type_conv: Copy the _oasis version constraints to opam.

In particular, the constraint on ocaml-version is utterly important
not to upgrade a 3.12 system.
Merge branch 'master' of git://github.com/OCamlPro/opam-repository
Add ocaml-sqlexpr.0.5.5
Merge pull request #802 from Chris00/master

type conv 109.28.00 should not install when using OCaml 3.12