Update gitlab-markup to 1.5.1 again
gitlab-markup
was accidentally reverted to 1.5.0 by https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/8537/diffs#note_21332125.
gitlab-markup
was accidentally reverted to 1.5.0 by https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/8537/diffs#note_21332125.