Conditional gemspecs never worked anyway.

Here's a good explanation of the problem.
  http://www.programmersparadox.com/2012/05/21/gemspec-loading-dependent-gems-based-on-the-users-system/

What this means is that this no longer installs cleanly on OSX < 10.9.

For that to work, you will need to manually install older versions of
the nokogiri and redcarpet gems before installing showoff.