When loading fixtures, do not trigger audits.

When testing in Django, the audits are automatically triggered. This
allows users to load fixtures without unnecessarily triggering audits.

See the keyward argument 'raw' here:
https://docs.djangoproject.com/en/1.6/ref/signals/#django.db.models.signals.pre_save

Also see previous unmerged PR from October 2013, leandrosouza/django-simple-audit#6