travis for 4.00.1, 4.01.0

Simple travis settings to build merlin on supported versions of OCaml for PR. It's a nice smoke test for external contributions and it does get annoying building on different compilers.

One of the committers @def-lkb @trefis @asmanur will need to enable CI for merlin on https://travis-ci.org/ and once this PR is merged travis will kick off and report the status. I've done this for my own fork and it worked fine.

Note that you might also want to add the test target in the shell script.