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: Id7e1130333e71acca7120fc6dd99ae4152e48a86
Reviewed-by: Iikka Eklund <iikka.eklund@digia.com>