Use musicbrainz5, drop earlier versions

Update sound-juicer to use version 5 of musicbrainz.  This is
mostly a straight substitution (MUSICBRAINZ4→MUSICBRAINZ5,
Mb4→Mb5, and so on).
routines:

Drop conditional support for version 3 of the API, leaving 5
only.

https://bugzilla.gnome.org/show_bug.cgi?id=676749