add libunity-core -> unity-common dep (LP: #1079132)
do not have unity-common dep on compiz-gnome: the migration code is only
ran if compiz-gnome is installed, but compiz-gnome is pulled by compiz
directly. We don't care if the migration code isn't executed. Fixes: https://bugs.launchpad.net/bugs/1079132.
Approved by Łukasz Zemczak.