Use forked github-markup gem
To use the forked version, we must undo some patching we did before.
I'm preserving one patch that changes the interpreter from `python2` to `python3` so that we don't change the required libraries for current source installs. This will let us release the change in a patch release instead of waiting for `8.13`.
Related issue: gitlab-ce#21696
See merge request !1015