Use forked github-markup gem to enable python3 support with omnibus

What does this MR do?

Changes the version of github-markup gem we are using, to be the forked one with the changes to enable python3 support. The change is backward compatible with users using Python 2, and will only be running with Python 3 in Omnibus (with the help of a custom patch we apply there).

Are there points in the code the reviewer needs to double check?

Make sure backward compatibility isn't broken for source installs.

For the release manager

You need to make sure you pickup into stable this merge request as well (for omnibus): omnibus-gitlab!1015 (merged) as it's part of the fix, otherwise package will fail to build.

Why was this MR needed?

We had some regressions with .rst rendering that were fixed, but still some issues with encoding persisted (this MR will fix that). See: https://gitlab.com/gitlab-org/gitlab-ce/issues/21696

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

• gitlab-org/gitlab-ce#21696
• gitlab-org/gitlab-ce#22748
• gitlab-com/support-forum#1097