Improve markup gems to prevent 500 errors

Created by: Razer6

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 (closed)
• #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