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)