Remove gemspec dependency on `git`

Shelling out is unecessary, especially since no .rb files are
gitignore'd.

Using the same thing I did for `overcommit`:

  https://github.com/causes/overcommit/blob/master/overcommit.gemspec#L20-L22

Change-Id: I3fb732dc5e5c59cd618d34e7373084fcf9c238b3
Reviewed-on: https://gerrit.causes.com/23262
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>