Fix failing Travis CI on Mac (#4086)

* Fix failing Travis CI on Mac

Use Homebrew Addon + latest Mac image

* Use long command for pytest

* Downgrade OSX image to xcode9.3, to use Java 8

* Install pytest in Python 2 environment

* Remove clang-tidy from Travis

tests/travis/setup.sh

@@ -1,10 +1,5 @@
 #!/bin/bash
 
-if [ ${TRAVIS_OS_NAME} == "osx" ]; then
-    brew update
-    brew install graphviz
-fi
-
 if [ ${TASK} == "lint" ]; then
     pip install --user  cpplint 'pylint==1.4.4' 'astroid==1.3.6' 
 fi