Remove transitive dependency on gitlab-grit

gitlab_grit has been replaced by Rugged throughout the gitlab-ce codebase (it no longer appears in Gemfile). However, it still appears in Gemfile.lock - because gitlab-flowdock-git-hook 1.0.1 still depends on it, and that gem is in the Gemfile.

https://github.com/bladealslayer/flowdock-git-hook/issues/2 exists to see about removing gitlab_grit from the gitlab- fork of https://github.com/flowdock/flowdock-git-hook

We could consider:

• Fixing the fork to use rugged
• Submitting a fix to use rugged to flowdock
• Producing our own integration
• Removing flowdock integration