Improve markup gems to prevent 500 errors

#### What does this MR do?

This PR add support mor missing markup-gems required by `github-markdown`. Gem versions are taken from https://github.com/github/markup/blob/master/Gemfile.

Additional, this PR disables markup for `pod` files, because an external tool is required (`perl`).

#### Why was this MR needed?

Currently, when accessing a `(textile, mediawiki, creole)`-file a 500 error is raised, because of missing gems.

#### What are the relevant issue numbers / Feature requests?

This PR fixes following issues:

* #7143
* #7377
* https://gitlab.com/gitlab-org/gitlab-ce/issues/412
* http://feedback.gitlab.com/forums/176466-general/suggestions/3811439-anyway-to-allow-mediawiki-markup-in-the-wiki-page