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: I52efbaa437979b4f01e0c10cbb5d09021e8cc39f
Reviewed-by: Iikka Eklund <iikka.eklund@digia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>