remove unecessary sed hiding proper error if docker not found

Follow-up to https://github.com/michaelsauter/crane/commit/0af9db88ba3ad7a0d33a26677851a0ce665c9003 (thanks - I wish I would have had time to do before you did it, sorry for that!). Verified on Linux and OS X: works when `docker` 1.0.0+ is within the path, show a proper message if not.