Per https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/10597#note_27752904 gitlab-ce!10597 introduces a simple versioning scheme for cached markdown, but there's a good chance that people changing the operation of banzai will forget that they need to increment the global version each time. Two alternatives to the simple scheme are discussed in that thread: version per banzai filter, or having a reference document that we hash to produce a version that will change automatically when banzai changes. Evaluate and implement one of them, or something better if we can think of it.