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: I8f899a9a9fb9e94b99a9250eb5eb2dbe2eed8fc5
Reviewed-by: Iikka Eklund <iikka.eklund@digia.com>