Fix scopes that use include for Rails 4
* Rails 4 creates new ActiveRecord::Relation objects when using scopes
  that use include.  In these scenarios, the passed klass is nil and
  that sends ::Relation#initialize_with_schema_associations into a
  tail spin.
* Added tests to spec/association_spec.rb for scopes both with and
  without include.  The former crashed under Rails 4.
* Added a guard to #initialize_with_schema_associations so that it
  doesn't attempt the call to #_load_schema_associations_associations
  if klass is nil.
* Since these updates are for Rails 4 compatibility, I updated the
  schema_plus dependency to require > 1.2.0 (which supports Rails 4).