[TravisCI] brew bundle --no-upgrade to avoid upgrade issues of unused formulae

.travis.yml

@@ -102,7 +102,7 @@ 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 bundle --file=tests/Brewfile
+  - travis_retry brew bundle --file=tests/Brewfile --no-upgrade
   # install neko
   - if [ ! $DEPLOY_NIGHTLIES ]; then
       brew install neko --HEAD;