Travis: simplify build matrix and improve script

Now testing only on different versions of OCaml, and with whatever is
the latest version of Core. Build fails on OCaml 3.12.1 for an unknown
reason. Don’t care to investigate. Script improvements are only
cosmetic.
opam package: add ocaml version constraint

Build is failing on OCaml 3.12.1 for an unknown reason. Might be
possible to make it work, but not worth it.