title: How do you specify a minimum Ruby version in a gemspec?
body: <p>I'm writing a gemspec for a new version of a gem that will now require Ruby 1.9. Previous versions of the gem were ok with Ruby 1.8, but now 1.9 will be required. Is there a way to cause the gem install for this version of the gem to fail with a warning for users who try to install it on Ruby 1.8?</p>