Make last names optional on Spree::Address

For example, we're integrating with Amazon Payments right now and
Amazon doesn't require two names, which breaks this validation. And
it's hard to remove an ActiveRecord validation at the app level.

core/app/models/spree/address.rb

@@ -5,7 +5,7 @@ class Address < Spree::Base
     belongs_to :country, class_name: "Spree::Country"
     belongs_to :state, class_name: "Spree::State"
 
-    validates :firstname, :lastname, :address1, :city, :country_id, presence: true
+    validates :firstname, :address1, :city, :country_id, presence: true
     validates :zipcode, presence: true, if: :require_zipcode?
     validates :phone, presence: true, if: :require_phone?
 

core/lib/spree/testing_support/factories/address_factory.rb

@@ -11,7 +11,7 @@
     end
 
     firstname 'John'
-    lastname 'Doe'
+    lastname nil
     company 'Company'
     address1 '10 Lovely Street'
     address2 'Northwest'