[CI] update brew commands

Author: h-2

.travis.yml

@@ -10,9 +10,8 @@ before_install:
  - test $(uname) = "Linux" && sudo apt-get update -y || true
  - test $(uname) = "Linux" && sudo apt-get install -qq -y g++-4.9 g++-5 g++-6 clang-3.8 clang-3.9 cmake cmake-data zlib1g-dev libbz2-dev || true
  - test $(uname) = "Darwin" && brew update || true
- - test $(uname) = "Darwin" && brew tap homebrew/versions || true
- - test $(uname) = "Darwin" && brew install gcc5 || true
- - test $(uname) = "Darwin" && brew install gcc6 || true
+ - test $(uname) = "Darwin" && brew install gcc@5 || true
+ - test $(uname) = "Darwin" && brew install gcc@6 || true
 matrix:
   include:
     - os: linux