[TravisCI] use OCaml 4.02.3 for Mac builds

See #6589.

.travis.yml

@@ -72,7 +72,10 @@ install_osx: &install_osx
   # Install haxe dependencies
   - brew uninstall --force brew-cask # https://github.com/caskroom/homebrew-cask/pull/15381
   - travis_retry brew update
-  - travis_retry brew install ocaml camlp4 zlib pcre;
+  - travis_retry brew install opam zlib pcre;
+  - opam init --comp 4.02.3 -y
+  - eval `opam config env`
+  - opam install camlp4 -y
   # Install neko
   - mkdir ~/neko
   - export NEKOPATH=~/neko