Define development packages: Preparation for Release 2.8.1.

necessary because:
 - https://github.com/4teamwork/ftw.treeview/pull/5
 - https://github.com/4teamwork/plonetheme.teamraum/pull/200
 - https://github.com/4teamwork/plonetheme.teamraum/pull/201

@lukasgraf please extend when it's necessary, and review