Made fixes:

- Added CMAKE_EXTRA_PARAMS environment to docker's container
  runs to enable -DENABLE_LTO=ON/OFF cmake option.

- Added CC/CXX environment to docker's container runs to set
  clang for cmake. Also the additional environment variables
  {CC,CXX}_FOR_BUILD were postponed, because we didn't
  run cross-compilation at the moment, for more info check:

    https://docs.travis-ci.com/user/languages/cpp/#choosing-compilers-to-test-against

- Changed LTO docker's image to 'debian-buster' due to LTO needed
  higher versions of packages, check for more information commit:

    f9e28ce4 ('Add LTO support')

- Fixed sources to avoid of failures on builds by GCC with LTO:

1)  src/box/memtx_rtree.c: In function ‘mp_decode_rect’:
    src/box/memtx_rtree.c:86:24: error: ‘c’ may be used uninitialized
      in this function [-Werror=maybe-uninitialized]
        rect->coords[i * 2] = c;
                            ^
    src/box/memtx_rtree.c:74:10: note: ‘c’ was declared here
      coord_t c;
              ^

2)  src/box/sql/func.c: In function ‘quoteFunc’:
    src/box/sql/func.c:1103:3: error: ‘b’ may be used uninitialized
      in this function [-Werror=maybe-uninitialized]
       sql_result_text(context, sql_value_boolean(argv[0]) ?
       ^
    src/box/sql/vdbeapi.c:217:7: note: ‘b’ was declared here
      bool b;
           ^

3)  src/box/tuple_update.c: In function ‘update_read_ops’:
    src/box/tuple_update.c:1022:4: error: ‘field_no’ may be used
      uninitialized in this function [-Werror=maybe-uninitialized]
        diag_set(ClientError, ER_NO_SUCH_FIELD_NO, field_no);
        ^
    src/box/tuple_update.c:1014:11: note: ‘field_no’ was declared here
       int32_t field_no;
               ^

4)  src/httpc.c: In function ‘httpc_set_verbose’:
    src/httpc.c:267:2: error: call to ‘_curl_easy_setopt_err_long’
      declared with attribute warning: curl_easy_setopt expects a long
      argument for this option [-Werror]
      curl_easy_setopt(req->curl_request.easy, CURLOPT_VERBOSE, curl_verbose);
      ^

5)  src/lua/httpc.c: In function ‘luaT_httpc_request’:
    src/lua/httpc.c:128:64: error: ‘MEM[(int *)&parser + 20B]’ may be used
      uninitialized in this function [-Werror=maybe-uninitialized]
      lua_pushinteger(L, (parser.http_minor > 0) ? parser.http_minor: 0);
                                                                    ^
    src/lua/httpc.c:67:21: note: ‘MEM[(int *)&parser + 20B]’ was declared here
      struct http_parser parser;
                         ^
    src/lua/httpc.c:124:64: error: ‘MEM[(int *)&parser + 16B]’ may be used
      uninitialized in this function [-Werror=maybe-uninitialized]
      lua_pushinteger(L, (parser.http_major > 0) ? parser.http_major: 0);
                                                                    ^
    src/lua/httpc.c:67:21: note: ‘MEM[(int *)&parser + 16B]’ was declared here
      struct http_parser parser;
                         ^

Close #4215

Fixes #4194.

It is important to have testing jobs that build the project with both
-Werror and -O2 to keep the code clean. -O2 is needed, because some
compiler warnings are available only after extra analyzing passes that
are disabled with lesser optimization levels.

The first attempt to add -Werror for release testing jobs was made in
da505ee7 ('Add -Werror for CI (1.10
part)'), but it mistakely doesn't enable -O2 for RelWithDebInfoWError
build. It is possible to fix it in this way:

 | --- a/cmake/compiler.cmake
 | +++ b/cmake/compiler.cmake
 | @@ -113,10 +113,14 @@ set (CMAKE_C_FLAGS_DEBUG
 |      "${CMAKE_C_FLAGS_DEBUG} ${CC_DEBUG_OPT} -O0")
 |  set (CMAKE_C_FLAGS_RELWITHDEBINFO
 |      "${CMAKE_C_FLAGS_RELWITHDEBINFO} ${CC_DEBUG_OPT} -O2")
 | +set (CMAKE_C_FLAGS_RELWITHDEBINFOWERROR
 | +    "${CMAKE_C_FLAGS_RELWITHDEBINFOWERROR} ${CC_DEBUG_OPT} -O2")
 |  set (CMAKE_CXX_FLAGS_DEBUG
 |      "${CMAKE_CXX_FLAGS_DEBUG} ${CC_DEBUG_OPT} -O0")
 |  set (CMAKE_CXX_FLAGS_RELWITHDEBINFO
 |      "${CMAKE_CXX_FLAGS_RELWITHDEBINFO} ${CC_DEBUG_OPT} -O2")
 | +set (CMAKE_CXX_FLAGS_RELWITHDEBINFOWERROR
 | +    "${CMAKE_CXX_FLAGS_RELWITHDEBINFOWERROR} ${CC_DEBUG_OPT} -O2")
 |
 |  unset(CC_DEBUG_OPT)

However I think that a build type (and so `tarantool --version`) should
not show whether -Werror was passed or not. So I have added
ENABLE_WERROR CMake option for that. It can be set like so:

 | cmake . -DCMAKE_BUILD_TYPE=RelWithDebInfo -DENABLE_WERROR=ON

Enabled the option in testing Travis-CI jobs with the RelWithDebInfo
build type. Deploy jobs don't include it as before.

Fixed all -Wmaybe-uninitialized and -Wunused-result warnings. A few
notes about the fixes:

* net.box does not validate received data in general, so I don't add a
  check for autoincrement IDs too. Set the ID to INT64_MIN, because this
  value is less probably will appear here in a normal case and so is the
  best one to signal a user that something probably going wrongly.
* xrow_decode_*() functions could read uninitialized data from
  row->body[0].iov_base in xrow_on_decode_err() when printing a hex code
  for a row. It could be possible when the received msgpack was empty
  (row->bodycnt == 0), but there were expected keys (key_map != 0).
* getcwd() is marked with __attribute__((__warn_unused_result__)) in
  glibc, but the buffer filled by this call is not used anywhere and so
  just removed.
* Vinyl -Wmaybe-uninitialized warnings are false positive ones.

Added comments and quotes into .travis.yml to ease reading. Removed
"test" word from the CentOS 6 job name, because we don't run tests on
this distro (disabled in the RPM spec).

Fixes #4178.

Fixes #4174.

Debian Wheezy is EOL since 31st May 2018, mirrors are disabled now.

Development branch is now named master, so
update conditions for extended extended checks on
Travis.
Also, set new repo name.

Fixes #4027.

Fixed Mojave Mac build with setting MACOSX_DEPLOYMENT_TARGET environment
variable for LuaJIT's Makefile. This variable specifies the minimum
version of OS X on which the target binaries are to be deployed.

The reason why we need to set MACOSX_DEPLOYMENT_TARGET to at least 10.6
is that 10.4 SDK (which is set by default in LuaJIT's Makefile) is not
longer included in Mac OS X Mojave 10.14.

See also https://github.com/LuaJIT/LuaJIT/issues/484

We already set -Wl,-macosx_version_min,10.6 (-macosx_version_min and
MACOSX_DEPLOYMENT_TARGET are synonymous), but it affects only a linker.
We possibly should remove -macosx_version_min, because it superseded by
MACOSX_DEPLOYMENT_TARGET, but it should be done with verification that
the original problem really fixed by the new way to set a deployment
target. This is not part of this patch.

Removed virtualenv usage in CI for OS X (so pip just installs packages
into a system), because OS X Mojave 10.14 does not offer virtualenv by
default.

Fixed #3797

These distributions are past EOL.

Disabled LTO builds, tarballs and packages building on short-term
branches.

Removed 'allow_failures' on coverage / debug build.

Replaced matrix expansion with the list of jobs (because Travis-CI
documentation says it does not support condition jobs with matrix
expansion).

Fixes #3755.

Closes: #3761

The idea behind this change is to have 2x (and maybe later 3x, 4x, ...)
bucket for alpha and beta releases. See more details about the release
process in the documentation: [1].

[1]: https://tarantool.io/en/doc/2.0/dev_guide/release_management/

Main branch was renamed to master.
Update .travis.yml accordingly.

Replaced targets generation using a matrix expansion + exclusion list
with the explicit targets list. Gave meagingful names for targets.

Fixes #3673.

We made a mistake when tagging 1.7.5, which confused git describe.
Now we have added more tags and the hack with removal can be deleted.

- Remove old versions of Fedora and Ubuntu
- Add Fedora 26 and Fedora 27

Patch removes Ubuntu Precise from Travis's testing matrix,
since it uses really outdated GCC v.4.6.x.

Switched from ubuntu zesty to artful, since zesty is EOLed on 13Jan2018

Remove all non-LTS versions of Ubuntu.

PackageCloud doesn't support Ubuntu Artful yet.

Follow up e3d97f6d

Closes #2805

Travis images are broken by design.

Ubuntu Trusty on Travis is buggy.

Fixes #2752

Ubuntu Trusty on Travis is buggy.

Fixes #2752

git describe picks 1.7.5 as a closest tag on 1.8 branch.
Temporary remove 1.7.5 tag to fix 1.8.x packages.