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: I4da67e288a68fd193cb451c233a2c7c4642d4725
Reviewed-by: Iikka Eklund <iikka.eklund@digia.com>