$ ruby -v
ruby 2.1.3p242 (2014-09-19 revision 47630) [x86_64-darwin13.0]

$ gem install redcarpet
Building native extensions.  This could take a while...
Successfully installed redcarpet-3.2.0
1 gem installed

$ gem install github-markup
Successfully installed github-markup-1.3.0
1 gem installed

$ irb
2.1.3 :001 > require 'github/markup'
=> true
2.1.3 :002 > GitHub::Markup.render('README.markdown', "* One\n* Two")
NameError: uninitialized constant GitHub::Markup::Markdown::RedcarpetCompat
    from /Users/bmsatierf/.rvm/gems/ruby-2.1.3@rails/gems/github-markup-1.3.0/lib/github/markup/markdown.rb:11:in `block in <class:Markdown>'
    from /Users/bmsatierf/.rvm/gems/ruby-2.1.3@rails/gems/github-markup-1.3.0/lib/github/markup/markdown.rb:44:in `call'
    from /Users/bmsatierf/.rvm/gems/ruby-2.1.3@rails/gems/github-markup-1.3.0/lib/github/markup/markdown.rb:44:in `render'
    from /Users/bmsatierf/.rvm/gems/ruby-2.1.3@rails/gems/github-markup-1.3.0/lib/github/markup.rb:23:in `render'
    from (irb):2
    from /Users/bmsatierf/.rvm/rubies/ruby-2.1.3/bin/irb:11:in `<main>'