updated validator to break build if rss feed invalid; plus refactoring

I've updated the xml validation to break the build if the rss is invalid, plus did a little drying out of the code.

Double check the gem spec file. I had to update the middleman and nokogiri versions, but not really sure how that's supposed to be handled. 

You should probably wait to actually publish a new gem version til I've actually fixed the rss feed validation errors, which I'll be working on next.