fix stand-alone validator - was using the 'rules' from the wrong scope

Hi!
I would love to use your gem in a little project of mine, which could become (at some point) part of [Diaspora*](https://github.com/diaspora/diaspora)... (do you release this gem under an official license?)
Anyway, I noticed the feature for self-contained validators as advertised in the Readme was not working correctly, and I hope to fix it with this pull request.

Let me know, if you need anything more for this to get merged ;)