Add Varnish 4 to testing matrix

Add Varnish 4 VCLs

Add Varnish repo

Use separate VCL set for Varnish 4 in tests

Execute APT commands as root

Execute APT commands as root

Travis runs on Ubuntu, not Debian, doh!

Re-add Varnish that fell out

Fix path to Varnish 4 VCLs

Fix string match