repository version can be selected over local .opam

[timbertson]

Trying to build a modified current_examples.opam (from ocurrent), it ends up selecting v0.4 from the public repo.

This is likely because I added another dependency, so it decided the public one was better (cheaper).

Adding an explicit version: field ensured opam2nix would only select that version, but then I also had to replace instances of the version opam variable with a literal "0.4".