PICARD-662: only consider collections of the "Release" type

After 2014-11-17 schema change, collection types were added by MBS-1059, causing an exception.
ian: is this documented somewhere (i mean collection types) ?
Imho such API break isn't much an issue, but if no server-side backwards compatibility workaround is done, we'll have to release Picard 1.3.1 soon (well, we'll have to do one anyway), and changes to collections have to be documented (or are they already?).

In the meantime, i can change the patch to skip any collection without "release-list" element.