PICARD-662: only consider collections having "release_list" element

After 2014-11-17 schema change, collection types were added by MBS-1059, causing an exception.

http://tickets.musicbrainz.org/browse/PICARD-662
Hm. The first version did have it with an underscore, but I changed that to a hyphen. Does python change it to an underscore, or was this tested against the version before I fixed that? It wasn't very long that it was there with the underscore, but...