Allow purge requests from local network

We need to do this so that Stagecraft can make purge requests when data-sets change.
Allow access to varnish from internal network

Stagecraft needs to be able to purge cached items form Varnish and
since Nginx on the frontend app servers drops PURGE requests Stagecraft
has to talk to Varnish directly.
Merge pull request #299 from alphagov/allow-purging-from-local-network

Allow purge requests from local network