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: I9bc9d2df9ce434453c652ac97bb786e6400d5af1
Reviewed-by: Iikka Eklund <iikka.eklund@digia.com>