Remove dependency on active_utils Validateable

The goal of this change is to begin breaking the dependency on active_utils and, in turn, ActiveSupport. We're now doing super simple validation in ActiveMerchant models, and there's a separate require to add Rails integration.

For now, using Rails-specific methods without explicitly requiring the integration will simply generate a deprecation warning. In ActiveMerchant 2.x the methods will have to be explicitly required.

@bslobodin @jduff @codyfauser this is the first in a series of pretty big changes, and will set the tone for what follows. I'm a big fan of incremental, backwards compatible changes vs. stop-the-world breaking changes, and that's reflected here. Would love to hear your thoughts on this patch and the approach in general before I embark on continuing to break dependencies and simplify the code.