Favoring Spree::Base for models instead of changing ActiveRecord::Base

Moving inclusion of Spree::Preferences::Preferable away from an engine-base inclusion to ActiveRecord::Base, and instead making it a Spree-specific Spree::Base inclusion.  This is to prevent the Prefereble module from bleeding outward and causing conflicts in other Engines which may have attributes called, at the least, 'preferred' on their models already.

All core test cases pass with this merge.