Use the json module as fallback for simplejson, and

[espenak]

remove the hard dependency on simplejson.

The motivation for this is that simplejson is not required, and it causes problems with Django 1.5 (see https://github.com/simplejson/simplejson/issues/37).

For most cases, the speedup of simplejson is not required, and if the system does not have a C compiler, simplejson is actually slower than the json module (see https://code.djangoproject.com/ticket/18023).