Errors in post-install scripts

[schmijos]

The last two lines in the post-install script cause errors if no dangling images are present:

Error response from daemon: invalid reference format: repository name must be lowercase
Error response from daemon: conflict: unable to delete be1b644615c9 (must be forced) - image is referenced in multiple repositories

Source: https://github.com/gliderlabs/herokuish/blob/master/contrib/post-install#L19-L23