Update rdoc to `~> 4.2`

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/2814

Depends on https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7260.

Mentioned in issue #2814 (closed)

Added 19 commits:

• 391fa58d...651bdd7c - 17 commits from branch master
• 4ba789cd - Remove unused gitlab:generate_docs Rake task
• 9dc3c4c0 - Update rdoc to ~> 4.2

Compare with previous version

Unmarked this merge request as a Work In Progress

Changed title: {-WIP: Update rdoc to ~> 4.2.0-} → {+Update rdoc to ~> 4.2+}

Status changed to merged

Mentioned in commit e4c05de7