Use native json module if available

I was running into some issues with newer versions of simplejson and django as described here:
https://docs.djangoproject.com/en/dev/releases/1.5/#simplejson-incompatibilities
and here:
https://docs.djangoproject.com/en/dev/releases/1.5/#django-utils-simplejson

The problem arises when there is another version of simplejson (>2.1) installed next to the usual django.utils.simplejson module.

Anyway, just a small patch to follow the recommendations of the django devs to use native json module instead of django.utils.simplejson.