Fix gemspec to not rely on `git`

This makes our deploys noisy, which makes me sad.

Related story:
  https://www.pivotaltracker.com/story/show/45264223
  ("Fix master_slave_adapter gemspec to not rely on `git`")

Change-Id: I944e723fd77d8819c456d35d347800a58e4efe45
Reviewed-on: https://gerrit.causes.com/19206
Tested-by: Aiden Scandella <aiden@causes.com>
Reviewed-by: Aiden Scandella <aiden@causes.com>