Use correct varnish version in default.vcl link
Allow Varnish to cache authorised requests

By default Varnish will not cache any requests with a Cookie or an
Authorization header. This change tells Varnish to lookup GET or HEAD
requests if the Authorization header is set.

This obviously raises the risk of sending a cached response to an
unauthorised client. This is mitigated by the backend app sending a Vary
header set to 'Authorized'.
Do not set stagecraft hostname in dev vcl

The nginx vhost for stagecraft has a server_name of
stagecraft.perfplat.dev which is the same host name that should be
coming into Varnish. So, let's just leave it be.
Merge pull request #310 from alphagov/varnish-caching

Varnish caching