Pin gitlab version to 4.15 due to a bug

Issue: gitlab#227786 (closed)

Due to a bug in the gitlab gem introduce in 4.16, we're getting the error method_missing': undefined method 'merge' in danger.

There is already an issue open: https://github.com/NARKOZ/gitlab/issues/574 and a proposed fix by @rymai https://github.com/NARKOZ/gitlab/pull/576.

However, to temporarily fix the issue and have our pipelines pass again, we can pin the gitlab dependency to 4.15 for now.