travis-ci: needs blind install for brew python2

Travis-ci failed on brew install python2 due to it was
already installed on OSX 10.13 Sierra, so it needs
to be installed without fails on its exists.

Follow up #4254

.travis.mk

@@ -49,7 +49,8 @@ test_ubuntu: deps_ubuntu
 
 deps_osx:
 	brew update
-	brew install python2 openssl readline curl icu4c --force
+	brew install openssl readline curl icu4c --force
+	python2 -V || brew install python2 --force
 	curl --silent --show-error --retry 5 https://bootstrap.pypa.io/get-pip.py | python
 	pip install -r test-run/requirements.txt