[TravisCI] use brew bundle to install brew formulae

It avoids getting "xxx already installed" error.

.travis.yml

@@ -129,8 +129,8 @@ install_osx: &install_osx
   # Install dependencies
   - travis_retry brew update --merge
   - brew uninstall --force brew-cask # https://github.com/caskroom/homebrew-cask/pull/15381
-  - travis_retry brew install opam ninja zlib pcre awscli;
-  - brew outdated cmake || brew upgrade cmake # we need a recent cmake to use CMAKE_OSX_DEPLOYMENT_TARGET
+  - travis_retry brew tap Homebrew/bundle
+  - travis_retry brew bundle --file=tests/Brewfile
   - export OPAMYES=1
   - opam init
   - opam install camlp4 sedlex ocamlfind xml-light extlib rope ptmap

tests/Brewfile

@@ -0,0 +1,6 @@
+brew "opam"
+brew "ninja"
+brew "zlib"
+brew "pcre"
+brew "awscli"
+brew "cmake"