Fix double callback of client.registration_d that occurs on disconnect

```
2014-06-05 16:09:36+0000 [-] Channel disconnect received
2014-06-05 16:09:36+0000 [-] TRACE: Deregistering client.
2014-06-05 16:09:36+0000 [-] Unhandled Error
        Traceback (most recent call last):
          File "/var/praekelt/vumi-go/ve/local/lib/python2.7/site-packages/twisted/application/app.py", line 392, in startReactor
            self.config, oldstdout, oldstderr, self.profiler, reactor)
          File "/var/praekelt/vumi-go/ve/local/lib/python2.7/site-packages/twisted/application/app.py", line 313, in runReactorWithLogging
            reactor.run()
          File "/var/praekelt/vumi-go/ve/local/lib/python2.7/site-packages/twisted/internet/base.py", line 1192, in run
            self.mainLoop()
          File "/var/praekelt/vumi-go/ve/local/lib/python2.7/site-packages/twisted/internet/base.py", line 1201, in mainLoop
            self.runUntilCurrent()
        --- <exception caught here> ---
          File "/var/praekelt/vumi-go/ve/local/lib/python2.7/site-packages/twisted/internet/base.py", line 824, in runUntilCurrent
            call.func(*call.args, **call.kw)
          File "/var/praekelt/vumi-go/ve/local/lib/python2.7/site-packages/eventsocket.py", line 197, in eventReceived
            return method(ctx)
          File "/var/praekelt/vumi-go/ve/local/lib/python2.7/site-packages/vxfreeswitch/voice.py", line 128, in onDisconnect
            self.vumi_transport.deregister_client(self)
          File "/var/praekelt/vumi-go/ve/local/lib/python2.7/site-packages/vxfreeswitch/voice.py", line 268, in deregister_client
            client.registration_d.callback(None)
          File "/var/praekelt/vumi-go/ve/local/lib/python2.7/site-packages/twisted/internet/defer.py", line 382, in callback
            self._startRunCallbacks(result)
          File "/var/praekelt/vumi-go/ve/local/lib/python2.7/site-packages/twisted/internet/defer.py", line 483, in _startRunCallbacks
            raise AlreadyCalledError
        twisted.internet.defer.AlreadyCalledError:
```