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: I3240876d8eea81f8313983e542ff94603d4768bf
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>