Bring back Oj in GitLab

In https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/17694#note_63821682, we came to the conclusion that Oj didn't improve performance.

In a benchmark for https://gitlab.com/gitlab-com/www-gitlab-com/merge_requests/16021/discussions.json, I see 11% time spent rendering JSON:

http://profiler.gitlap.com/20181120/8714c236-f927-457e-b3c7-0fe14e2af5e7.html.gz

I suspect Oj may be much faster than using the native Ruby JSON serializer:

https://github.com/ohler55/oj/blob/master/pages/Rails.md

/cc: @oswaldo, @godfat, @rspeicher