Update git2-rs version once it's out

[Nukesor]

Libgit2 released a new version 1.4 and the bindings currently break on systems that already migrated to that version.

git2-rs is planning to release a new version with the update to v1.4: rust-lang/git2-rs#811

Once this is done, I'll create a PR that'll simply bumps the version :)

[Nukesor]

I might also take a shot at replacing git2-rs with gitoxide, which would remove any future problems due to external library bindings.

[oknozor]

That's a great idea, unfortunately the clone functionality is not ready yet on gitoxide (see: Byron/gitoxide#303). But once it's ready it would be neat to switch to gitoxide :)