Upgrade to Rails 5

TODO:

• Update to Rails 5.0.x, make tests green, do QA and merge - https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12841
• Address problems https://gitlab.com/gitlab-org/gitlab-ce/issues/14286#note_15844255, https://gitlab.com/gitlab-org/gitlab-ce/issues/14286#note_32366152, https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13560
• Update to Rails 5.1, make tests green, do QA and merge

Benefits

1. with sockets, which are necessary for:
   • https://gitlab.com/gitlab-org/gitlab-ce/issues/4103 collaborative snippets
   • https://gitlab.com/gitlab-org/gitlab-ce/issues/4199 collaborative issues, MRs
   • live updating builds
2. It seems we need it for PG 10 support too https://gitlab.com/gitlab-org/gitlab-ce/issues/38222#note_41332032
3. Continue receiving bug fixes and security updates