Fix mismatch parallel dependency in Gemfile

The Gemfile specified version `~> 1.17.0` was mismatched with the
Gemfile.lock version `1.19.1`.

This seems have possibly occurred due to an issue with 2 different merge
requests changing this at the same time and being merged without
rebasing on each other:

- https://gitlab.com/gitlab-org/gitlab/merge_requests/20738
- https://gitlab.com/gitlab-org/gitlab/merge_requests/21886