Use system json instead of django.utils.simplejson

django.utils.simplejson was deprecated in Django 1.5
https://docs.djangoproject.com/en/dev/releases/1.5/#django-utils-simplejson

Note: This will break in Python versions <= 2.5
Merge pull request #1 from kevinschaul/master

Use system json instead of django.utils.simplejson