Bug 1247344 - Silence the HSTS subdomains check
Bug 1247344 - Silence the HSTS subdomains check

Since we cannot set `SECURE_HSTS_INCLUDE_SUBDOMAINS` to True whilst the
Vagrant and Travis SITE_URLs are fake subdomains of the stage/prod URLs,
and can't use HTTPS.

https://docs.djangoproject.com/en/1.8/ref/settings/#std:setting-SILENCED_SYSTEM_CHECKS

glenn124f/treeherder,tojonmz/treeherder,tojonmz/treeherder,glenn124f/treeherder,tojon/treeherder,kapy2010/treeherder,tojon/treeherder,edmorley/treeherder,kapy2010/treeherder,tojonmz/treeherder,tojonmz/treeherder,akhileshpillai/treeherder,glenn124f/treeherder,tojon/treeherder,KWierso/treeherder,glenn124f/treeherder,glenn124f/treeherder,kapy2010/treeherder,tojonmz/treeherder,tojon/treeherder,KWierso/treeherder,edmorley/treeherder,KWierso/treeherder,akhileshpillai/treeherder,kapy2010/treeherder,KWierso/treeherder,edmorley/treeherder,akhileshpillai/treeherder,glenn124f/treeherder,akhileshpillai/treeherder,edmorley/treeherder,kapy2010/treeherder,tojonmz/treeherder,akhileshpillai/treeherder,akhileshpillai/treeherder