Use twisted 16.2 from jessie-backports

New soledad packages now depend on Twisted 16.2.0 (see https://leap.se/code/issues/8412), so we need to pin twisted to get installed from jessie-backports. See https://leap.se/code/issues/8412#note-8 for the working preferences.d snippet

(from redmine: created on 2016-08-29, closed on 2016-10-25, relates #8412, relates #8287 (closed))