I had the same issue. A hint as to what’s going on is that some gems are being loaded from your home directory (/home/ronnie/gems) while others, in particular kramdown, are loaded from /usr/lib/ruby/vendor_ruby, which contains “customizations provided by the OS vendor” (https://stackoverflow.com/a/26974928/1224002). The trouble is, these “customizations” are in this case for an older, pre-2.0.0 version of Kramdown! Kramdown 2.0.0 removed the OrderedHash function.
One solution, which worked for me, is simply to remove the /usr/lib/ruby/vendor_ruby directory. Caveat emptor.