Replace JSONField from django-pgjson with native Django JSONField

[moshthepitt]

In Django 1.11 the postgres JSON field was added into core, see: https://docs.djangoproject.com/en/1.11/ref/contrib/postgres/fields/

We should use this instead of the JSONField provided by https://github.com/djangonauts/django-pgjson. Not only because it is better to use core Django whenever possible, but also because django-pgjson has not been update din two years and uses a deprecated (as of Django 1.10) Django function: djangonauts/django-pgjson#45