[trusty] Use Varnish 4.x instead of Varnish 3.0

[googletorp]

I think it would be a good idea to use Varnish 4.x instead of Varnish 3.0 for the new trusty box we are building.
For Drupal 8 sites, I think this can make a huge difference, since Varnish 4.x has a lot better support for some of the headers like no-cache that Drupal 8 also uses. In time if people want to develop and test sites, that uses cache tags for clearing varnish cache. Most likely such features/modules would only support Varnish 4.x