Fixed #427

Looks like people weren't always updating their config files and the new listen option wasn't getting populated, which was causing problems (like #427). This makes that config option optional.
Attempted fix to #389 (and many others)

Sounds like the latest CFPropertyList gem (2.2.0) breaks SiriProxy. I adjusted the runtime dependency to use 2.1.2 instead (which should work).

I'm away from home so I don't actually have the ability to test the fix, but hopefully someone can test it.
Merge branch 'cfpropertylist-fix' of github.com:plamoni/SiriProxy into cfpropertylist-fix