Update gollum repository location in the Gemfile.
Created by: DanKnox
Github just moved the Gollum repository to it's own organization. This commit updates the Gemfile to point to the new repo location.
I also added a specific ref pointer to a version that is known to work well just in case they slip up and break their master branch right at the same time someone happens to be installing or updating Gitlab. This is about the safest we can get until they put out the next release that resolves the Pygments dependency issue.