Provenance not being stored during ingest

At https://github.com/OpenAddressesUK/ernest/blob/master/lib/jobs/create_address.rb#L29, we create a Source model for the provenance, but a Source requires it's OWN provenance, so needs to have an Activity added here with the date at which is was downloaded, or something. We have worked around this for now, but it needs to be fixed in beta when we are storing richer provenance chains.
