Getting the sha via the Heroku API will fail after 200 commits

[pezholio]

At the moment, we get the commit sha for the provenance using the Heroku API, which is a bit messy, but due to the lack of a filesystem, we can't use Git to get the current SHA:

https://github.com/OpenAddressesUK/distiller/blob/master/lib/helpers.rb#L12

Also, this will break eventually as the Heroku API gem only returns the first 200 commits. We probably need to fix this before we hit 200 commits.