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.

This fixes the comments in http://get.docker.io/ and presumably http://test.docker.io/

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