Revert "use merge for brew update (should be faster)"

This reverts commit 249391f26a8b544a84c9bfc87a9eeccc598e8683.

.travis.yml

@@ -102,7 +102,7 @@ install_linux: &install_linux
 install_osx: &install_osx
   - (nvm ls 8 || nvm install 8) && nvm use 8
   # Install dependencies
-  - travis_retry brew update --merge
+  - travis_retry brew update
   - brew uninstall --force brew-cask # https://github.com/caskroom/homebrew-cask/pull/15381
   - travis_retry brew bundle --file=tests/Brewfile --no-upgrade
   # install neko