sync.profile: Point dependencies to an empty string

This means that we are going to use the same branch name for the
dependencies as the branch of the repository under test.

If we are testing the 'stable' branch, all dependencies will point
to the 'stable' branch as well.

Change-Id: Ia57b19490beaab0ab551620c5a033aaf0189a5b0
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>