Allow stable updates when using the beta channel

For now, a client only gets the update from the channel it is subscribed to: release or beta. It is normal for someone on the release channel to not get beta updates, but when someone is on the beta channel, he should get the stable updates if they are more recent.

A direct consequence of the current situation is that there can be no available versions when in the BaseUpdater._force_downgrade() method, and there is an unhandled exception thrown by versions.keys().