Add support for new https://gem.coop/ Rgem repo

[pombredanne]

This is an alternative repo that should yield PURL with a repository_url qualifier.
The structure is the same otherwise, with an index at https://gem.coop/specs.4.8.gz

[pombredanne]

@indirect actually something that would be awesome is to publish the gem.coop metadata in a git repo, making it instantly widely available.