Add Ruby 2.0 to the Travis test matrix

Ruby 2.0 was recently released and is production ready:

  http://www.ruby-lang.org/en/news/2013/02/24/ruby-2-0-0-p0-is-released/

We don't officially support Ruby 2.0 nor do we intend to do so in the
near future, so adding it as an allowed failure.