Fix the .toplevel() code

Broken by: c035954

Author: vdboor

polymorphic_tree/managers.py

@@ -31,4 +31,4 @@ def toplevel(self):
  """
  # By using .all(), the proper get_query_set()/get_queryset() will be used for each Django version.
  # Django 1.4/1.5 need to use get_query_set(), because the RelatedManager overrides that.
- return qs.all().toplevel()
+ return self.all().toplevel()