Revert 6fc6e4c17f5b2d29291606740a7e6bc19902b427 to set the min OCaml version

In an utterly baffling bug, setting OCamlVersion in _oasis seems to
generate a completely different of autogenerated files that don't
work.  We can enforce the minimum restriction in OPAM instead.
enforce minimum ocaml version as 4.01.0
Merge pull request #190 from avsm/master

Enforce version restriction through OPAM instead of OASIS