changed env for macOS

.travis.yml

@@ -16,7 +16,7 @@ matrix:
     - os: osx
       osx_image: xcode8
       env:
-        - MATRIX_EVAL="brew install gcc && CC=gcc-6 && CXX=g++-6"
+        - MATRIX_EVAL="brew cask uninstall oclint && brew install gcc@6 && CC=gcc-6 && CXX=g++-6"
 
 before_install:
   - eval "${MATRIX_EVAL}"