Revert "Upgrade to Varnish 4.0"

This reverts commits fe8a534dd4a2119a14fed722eec3fb095071b45c and 1c00e552a76c7e52167d202e3abfe704a5e0a12b.
Don't apt-get update when not running Varnish 4.0
Install Varnish Agent from private repository
Expose varnish-agent with SSL on port 6086
Increase size of /var/lib/varnish from 128MB to 256MB