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: Ibb0cf31b90d04f280691859d66486803ff58cc78
Reviewed-by: Iikka Eklund <iikka.eklund@digia.com>