Fiz use of naive datetime on tests

pythonpro/cohorts/tests/test_cohorts.py::test_no_access
  /home/travis/virtualenv/python3.7.1/lib/python3.7/site-packages/django/db/models/fields/__init__.py:1423: RuntimeWarning: DateTimeField CohortStudent.added received a naive datetime (2018-06-12 00:00:00) while time zone support is active.
    RuntimeWarning)