Add Ruby 2.0.0 to Travis CI build

Now that Ruby 2.0.0 is out, it's time to officially support it! (I tested and the specs pass.)

It would also be great to have a new release. I learned the hard way (through hours of debugging) that workflow 0.8.7 doesn't include the private/protected callback method fixes, and hence doesn't work in Ruby 2.0.0.