Use system json module, not Django simplejson

Fix #1383

Thanks to @willingc for pointing out the simplejson deprecation,
and to @ehashman for filing the bug.

I've tested this patch on linode2.openhatch.org, and it generates
a data snapshot without crashing.
+1 on the changes and glad it is working again. Not a show stopper but probably should edit the comment to remove confusion re: simplejson since it no longer applies.