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: I87e02c9d323b524898d93fb5d1d4387218a984c5
Reviewed-by: Janne Anttila <janne.anttila@digia.com>