Remove deprecated github-markdown markup
Remove deprecated github-markdown markup

From [RubyGems](https://rubygems.org/gems/github-markdown/versions/0.6.9):

> THIS GEM IS NOT MAINTAINED AND NOT SUPPORTED.

Also, the GitHub repository, maintained by GitHub, was removed.

The link is now a redirect, it doesn't point to the GFM page anymore.
rgroux/gollum,rgroux/gollum,gollum/gollum,rgroux/gollum,gollum/gollum,repotag/gollum,gollum/gollum,rgroux/gollum,gollum/gollum,gollum/gollum,repotag/gollum,ProgramFan/gollum,rgroux/gollum,sunnyone/gollum,ProgramFan/gollum,ProgramFan/gollum,repotag/gollum,repotag/gollum,jinhu/gollum,jinhu/gollum,sunnyone/gollum,sunnyone/gollum,jinhu/gollum,sunnyone/gollum,ProgramFan/gollum,repotag/gollum,jinhu/gollum