Update Songbird to 0.4.0

[hal7df]

It's not entirely clear, but there's some evidence to suggest that the version of Songbird used by bardcast doesn't work particularly well with the Discord API in its current state. Songbird does have a 0.4.0 release out, but it involves a fairly significant breaking change to how audio streams are handled.

The new library version includes new RawAdapter and AsyncAdapterStream types, either of which might be useful for bardcast.