Don't use boot2docker ISO on disk

This was a hack for the DockerCon demo. We do want some way of
caching the boot2docker image, but this certainly isn't the way.

Signed-off-by: Ben Firshman <ben@firshman.co.uk>
Merge pull request #53 from bfirsh/do-not-use-boot2docker-iso-on-disk

Don't use boot2docker ISO on disk