sync.profile: Point dependencies to 'refs/heads/stable'

We should test stable branches against stable branches only.
At some point we should automate the merges from dev->stable->release
and decide how to handle possible merge conflicts.

This is good enough for the time being.

Change-Id: I42f68dd5f073a466cecbe9d811a0152af7256895
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Janne Anttila <janne.anttila@digia.com>