Pin `gitlab`, `danger, and `danger-gitlab` gems in Dockerfile.danger
Was: Revert pinning gitlab to 4.15
With !308 (merged) being merged, we are pinning the gitlab gem to 4.15. We want to revert this MR after https://github.com/NARKOZ/gitlab/pull/576 got merged and a new gem version got published.
Edited by Lin Jen-Shin