Make sure tests can run in Django 1.11

polymorphic/tests/__init__.py

@@ -204,6 +204,7 @@ class MROBase2(MROBase1):
 
 
 class MROBase3(models.Model):
+    id2 = models.AutoField(primary_key=True)  # make sure 'id' field doesn't clash, detected by Django 1.11
     objects = PolymorphicManager()