Update AddressField for Django 2.0

Overloaded init needs to set on_delete

virtual_only has been renamed to private_only
django/django@c339a5a

Refs furious-luke#76

address/models.py

@@ -291,10 +291,12 @@ class AddressField(models.ForeignKey):
 
     def __init__(self, *args, **kwargs):
         kwargs['to'] = 'address.Address'
+        kwargs['on_delete'] = models.CASCADE
         super(AddressField, self).__init__(**kwargs)
 
-    def contribute_to_class(self, cls, name, virtual_only=False):
-        super(ForeignObject, self).contribute_to_class(cls, name, virtual_only=virtual_only)
+    def contribute_to_class(self, cls, name, private_only=False):
+        super(ForeignObject, self).contribute_to_class(
+            cls, name, private_only=private_only)
         setattr(cls, self.name, AddressDescriptor(self))
 
     # def deconstruct(self):