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.
Remove kombu 1.4.3
Adding upgraded kombu, 3.0.22
Remove amqplib 1.0.2
Add amqp 1.4.6
Fixes comment for recent change to json
Removing settings link from nav bar