Increase required Go version in Build-Deps

We need at least go 1.11 due to a sort.Slice in the vendored deps.

debian/control

 Source: systemd-docker
 Priority: optional
 Maintainer: ale <ale@incal.net>
-Build-Depends: debhelper (>=9), dh-golang, golang-go (>= 1.6)
+Build-Depends: debhelper (>=9), dh-golang, golang-go (>= 1.11)
 Standards-Version: 3.9.6
 
 Package: systemd-docker