Fix markdown tests regarding latest redcarpet, rdiscount and kramdown

Hi,

I believe that markdown tests should be run with the respect to which markdown gem is actually loaded (redcarpet > rdiscount > kramdown). I was able to run the tests just fine with kramdown, but not with redcarpet nor rdiscount -- both are slightly different and don't offer `:auto_ids` option.

This should fix one of the things regarding the Ruby 2.0.0 issue [1]. Maybe some nicer/dried way of determining which gem is going to be used can be later introduced.

[1] https://github.com/slim-template/slim/issues/371