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: Ia4afe46fdc798f3e037fdc84cb48886ee60d5112
Reviewed-by: Iikka Eklund <iikka.eklund@digia.com>