Fixed use of deprecated Query.aggregates

In Django 1.8 [`Query.aggregates` is replaced by `annotations`](https://docs.djangoproject.com/es/1.9/releases/1.8/#aggregate-methods-and-modules).

Using aggregates currently raises a `RemovedInDjango20Warning`. [Django's source](https://github.com/django/django/blob/1.8/django/db/models/sql/query.py#L192)

rest_framework/filters.py

@@ -245,7 +245,7 @@ class OrderingFilter(BaseFilterBackend):
             ]
             valid_fields += [
                 (key, key.title().split('__'))
-                for key in queryset.query.aggregates.keys()
+                for key in queryset.query.annotations.keys()
             ]
         else:
             valid_fields = [