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
It seems picard is replacing "-" with "_" due to https://github.com/musicbrainz/picard/blob/master/picard/webservice.py#L99 (and following lines).