First pass at using rugged

@robolson @square/kochiku-contributors this is a first pass at switching out one of the cocaine calls for rugged.

@cheister I am not sure if I have maintained the order of the original method?

I am not thrilled about `repo.last_commit.parents.first.parents.first.oid` but I could not find an easy way to grab the first commit from rugged. Do you guys see something obvious that I missed?

Also wasn't super thrilled about locking to development (basically master) of rugged but the last release is pretty far behind development: https://github.com/libgit2/rugged/compare/7f408b84908152f1480234b52f49951f35261eca...development