Implement a post-download hook for code repositories.

Use the post-download hook to obtain the LilyPond version
from the VERSION file as before, but only after we have
successfully cloned the git repository.

Fixes issue 4: git download assumes dir already exists
https://github.com/janneke/gub/issues/4
Rename post_download_hook to bind_version.
Merge pull request #4 from jrioux/lily-build

Implement post-download hook for git repository.