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: I81ec5d9ef660ebd3f69bee74a95b8af0164cdeba
Reviewed-by: Iikka Eklund <iikka.eklund@digia.com>