downloader: CLI breaks when record has no files

[avivace]

The current version of the package breaks when no "files" key is found in the metadata object.
E.g.

$ cernopendata-client download-files --recid=60

This should be fixed by 465072a but the current release (0.2.0) doesn't include this commit. Perhaps a bugfix minor release? This would allow modules using cernopendata-client as dependency to avoid shipping the git version