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]: https://github.com/crosbymichael/docker/blob/267ca39921c35826ccbdb84fbbc0690bfef385d7/runtime/graphdriver/driver.go#L40-L46

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