Add cgroup-bin dependency to our Ubuntu package

Since cgroup-bin is only "recommended" by the lxc package on Ubuntu, but is necessary for having the proper cgroups mounted for Docker to function, this makes some sense for us to add separately.

Fixes #2990

hack/make/ubuntu

@@ -96,6 +96,7 @@ EOF
 		    --depends lxc \
 		    --depends aufs-tools \
 		    --depends iptables \
+		    --depends cgroup-bin \
 		    --description "$PACKAGE_DESCRIPTION" \
 		    --maintainer "$PACKAGE_MAINTAINER" \
 		    --conflicts lxc-docker-virtual-package \