Fixed unwanted manager replacement in Django 1.11 projects.

Django 1.11 uses the old manager inheritance system, unless it's
overwritten with manager_inheritance_from_future. With a class layout
like:

  PolymorphicModel  (abstract)
    PolymorphicMPTTModel  (abstract)
      GenericCustomer  (concrete, has objects = ...)
        CustomerGroupBase (abstract, has objects = ...)
          Partner (concrete, no manager)
            BranchPartner  (concrete, no manager)

The last level gets a normal Django Manager instead of the polymorphic
manager. Because the PolymorphicModel had a base_objects manager, this
was typically used as _default_manager. Now that the default manager is
no longer affected, it's also easier to detect why the "objects" doesn't
get the proper manager type. Using "manager_inheritance_from_future" is
recommended instead to have both the right behavior and forward
Django 2.x compatibility.

polymorphic/tests/models.py

@@ -191,7 +191,9 @@ class MROBase1(ShowFieldType, PolymorphicModel):
 
 
 class MROBase2(MROBase1):
-    pass  # Django vanilla inheritance does not inherit MyManager as _default_manager here
+    class Meta:
+        # Django 1.x inheritance does not inherit MyManager as _default_manager here
+        manager_inheritance_from_future = True
 
 
 class MROBase3(models.Model):
@@ -200,7 +202,8 @@ class MROBase3(models.Model):
 
 
 class MRODerived(MROBase2, MROBase3):
-    pass
+    class Meta:
+        manager_inheritance_from_future = True
 
 
 class ParentModelWithManager(PolymorphicModel):