rocks: bump 3.1.3 version

The current version of Luarocks used in tarantool has a
security vulnerability:
https://github.com/luarocks/luarocks/pull/1019

This issue is fixed in release 3.1.3, commit:
6ffa8cbe2bcdea2c1ee0f4b32291d179210707b6

Part of https://github.com/tarantool/security/issues/12

NO_DOC=bugfix
NO_TEST=bugfix
NO_CHANGELOG=bugfix