Switch back to github linguist

Also bumps rugged to 0.23.3. This is in preparation to support gitlab-org/gitlab-ce!637.

Also closes https://github.com/gitlabhq/gitlab_git/issues/62.

rugged changes:

• The :safe_create flag was removed from Repository#checkout_tree. You can use :create in combination with :recreate_missing instead.

• Rugged::Remote instances are now immutable. Remote#clear_refspecs and Remote#save were removed without replacement. Remote#url= and Remote#push_url= were removed and replaced by RemoteCollection#set_url and RemoteCollection#set_push_url.