Deprecate django.utils.simplejson in favor of Python 2.6's built-in json module

Since `django-rest-framework` doesn't support Python 2.5, we can now rely on the json module being available in Python's standard library.

Django maintainers [recommend](https://docs.djangoproject.com/en/dev/releases/1.5/#system-version-of-simplejson-no-longer-used) to use it from now on.