Provide DJANGO_SETTINGS_MODULE when vaidating YAML

tastypie.serializers depends on django.core.serializers.json,
which depends on django.db.models, which will only import
if the settings object is properly configured, so before this
commit, we would crash with:

django.core.exceptions.ImproperlyConfigured: Requested setting DATABASES, but settings are not configured. You must either define the environment variable DJANGO_SETTINGS_MODULE or call settings.configure() before accessing settings.

See http://inside.openhatch.org/crawl-logs/scrapy.2014-09-07.atP3.log
(which may be deleted due to log rotation) for the full traceback.
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.
Merge pull request #1419 from paulproteus/validate-yaml-with-django-settings

Provide DJANGO_SETTINGS_MODULE when vaidating YAML
Merge pull request #1420 from paulproteus/system-json-module

Use system json module, not Django simplejson