Do CreditCard attribute cleanup at assignment

The benefit of doing cleanup actively at assignment is that it does not
assume a specific lifecycle for the CreditCard.
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.