Django 1.6 support

CHUNK_SIZE has been removed from Django, so set it ourself if it can't
be imported.

polymorphic/query.py

@@ -15,7 +15,11 @@ from .query_translate import translate_polymorphic_field_path
 
 # chunk-size: maximum number of objects requested per db-request
 # by the polymorphic queryset.iterator() implementation; we use the same chunk size as Django
-from django.db.models.query import CHUNK_SIZE               # this is 100 for Django 1.1/1.2
+try:
+    from django.db.models.query import CHUNK_SIZE               # this is 100 for Django 1.1/1.2
+except ImportError:
+    # CHUNK_SIZE was removed in Django 1.6
+    CHUNK_SIZE = 100
 Polymorphic_QuerySet_objects_per_request = CHUNK_SIZE