install.sh (get.docker.io) aufs comment updated.

devicemapper has landed, but the TODO hasn't been actioned presumably
because aufs is still preferred over devicemapper when available[1].

Comment updated accordingly.

Citation [1]: 267ca39921/runtime/graphdriver/driver.go (L40-L46)

Docker-DCO-1.1-Signed-off-by: Paul Annesley <paul@annesley.cc> (github: pda)

hack/install.sh

@@ -85,7 +85,7 @@ case "$lsb_dist" in
 			fi
 		}
 		
-		# TODO remove this section once device-mapper lands
+		# aufs is preferred over devicemapper; try to ensure the driver is available.
 		if ! grep -q aufs /proc/filesystems && ! $sh_c 'modprobe aufs'; then
 			kern_extras="linux-image-extra-$(uname -r)"