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.