title: How do I add conditional rubygem requirements to a gem specification?
body: <p>Is it possible to add a gem dependency only if the person is using a certain version of ruby?</p>
  
  <p><strong>Background:</strong> I'm working on a <a href="http://github.com/agrimm/chaser" rel="nofollow noreferrer">fork of a project</a> that uses Test::Unit::Autorunner and the like. They are part of the standard library in ruby 1.8, but aren't part of the standard library in 1.9.1, and is instead in the "test-unit" gem. I want to add a dependency that says that if someone's using ruby 1.9.1 or later, install the "test-unit" gem, but if they're using 1.8 or earlier, they don't need to install anything.</p>