[commhistory-daemon] Don't attempt to set ring account presence

Mission control misbehaves when ring's presence is set to offline,
dropping all channels and future incoming channels.

I'm looking into a telepathy-side fix to prevent any presence changes to
ring, but commhistory-daemon shouldn't attempt to set it in response to
global presence changes anyway.
Merge pull request #32 from special/ring-presence

[commhistory-daemon] Don't attempt to set ring account presence