Upgrade gitlab-markup gem

What does this MR do?

Upgrade the gitlab-markup gem to pull in https://gitlab.com/gitlab-org/gitlab-markup/merge_requests/6

Documentation was already updated in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13900

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together