use merge for brew update (should be faster)

.travis.yml

@@ -125,7 +125,7 @@ install_osx: &install_osx
     fi
   # Install dependencies
   - brew uninstall --force brew-cask # https://github.com/caskroom/homebrew-cask/pull/15381
-  - travis_retry brew update
+  - travis_retry brew update --merge
   - travis_retry brew install opam ninja;
   - export OPAMYES=1
   - opam init