Change github-markdown dependency to a free licensed parser

[balasankarc]

Hi,
I am packaging html-pipeline for Debian and I just found out that it has the gem 'github-markdown' as a dependency. That gem is not released under any Free (and Open Source) license and I don't think will ever be (based on what I heard from Github developers), which makes it unable to be adapted to Debian. This, in turn, blocks me from packaging html-pipeline.

So, can you please change this dependency to some other parsers like redcarpet. It would be a great help.

Thanks.