Upgrade to virtus 1

Hey,
Thanks for the gem it's pretty awesome and made our interaction with PayPal api much easier :+1: 
However, we found that dependency of pp-adaptive on old version of virtus blocks us from updating our other gems. For example grape 0.6.1 requires virtus >= 1.0.0. 
We have updated gemspec and fixed all tests after everything broke down :\ Please, have a look and let us know what you think. I don't think it's possible to make it work for both virtus 0.5.5 and 1.0.0 at the same time so you might want to release this as major version bump.
Thanks.