Update git2 dependency and bump version

[steveklabnik]

This crate won't even build today, given that the dependencies now produce a divergent requirement on libz:

native library `z` is being linked to by more than one package, and can only be linked to by one package

  libz-sys v0.1.9
  libz-sys v1.0.0

Updating git2 fixes this.

I also included a commit to release a new version with this update, that'd be great if you could :)

[frewsxcv]

Thanks! I'm out right now but I'll merge this later and publish a new version

[steveklabnik]

Cool. No real rush :)

[frewsxcv]

Published. Thanks again!