[TravisCI] ptmap is OCaml 4.06 compat. now

.travis.yml

@@ -103,7 +103,6 @@ install_osx: &install_osx
   - opam init
   - eval `opam config env`
   - opam update
-  - opam pin add ptmap https://github.com/andyli/ptmap.git#ocaml406 --no-action # https://github.com/backtracking/ptmap/pull/8
   - opam pin add haxe . --no-action
   - opam install haxe --deps-only
   # install neko