merge 3.18.4 into default server/test/unittest_migractions fails due to interaction between new constraint test (from 3.18) and composite deletion change (in 3.19).

#!/bin/sh -e
if [ "$1" = "purge" ] ; then
        update-rc.d cubicweb remove >/dev/null
fi
 
#DEBHELPER#
 
exit 0