[TravisCI] use OCaml 4.06 anyway since we do not want to build from source...

.travis.yml

@@ -25,6 +25,8 @@ env:
     - OPAMYES=1
     # make variables
     - ADD_REVISION=1
+    # OCaml 4.06 compat. workaround
+    - OCAMLPARAM="safe-string=0,_"
     # nightly builds submit
     - secure: "UoGjYvQqt66GWmeLC4Pih1iue5AufVgW8XQOd2Bx839NN/2mQQ9bD1HuONJe+taWBJ+PHInkAjYROYYaiCQUA7B1SXs3oQD7Og6arVcR7kY7XOdAQ2t8ZkxJHTnuYGWW/2gNFBESv+3H17bkXG4rzaSn2LV5PJLOxSjw0ziBUMY="
     - secure: "ugpxt+zeYiAiMYKLK96f5TLSxbQAtmDWiumdwaLHl88fIUeefxJJPIF1Xm0AHeYEJE7sD8dLE1dMbRSzOpXFfTmJoQZv19Wjv+2N5B+DaabKjGj1nZG7q3blGa3nUYzWVfFNFiIpM9c5fvW8yiUFzacZE5itEY8+lZQeGsNh+WQ="
@@ -100,7 +102,7 @@ install_osx: &install_osx
   - brew uninstall --force brew-cask # https://github.com/caskroom/homebrew-cask/pull/15381
   - travis_retry brew tap Homebrew/bundle
   - travis_retry brew bundle --file=tests/Brewfile
-  - opam init --compiler=4.05.0
+  - opam init
   - eval `opam config env`
   - opam update
   - opam pin add haxe . --no-action