Fix acquiring source hop in routing table dispatcher when billing in the source

This issue was discovered because events weren't being routed correctly during a load test. Example traceback:

```python
2014-02-18 14:39:28+0200 [VumiRedis,client] 'Error routing message for billing_dispatcher_ro'
2014-02-18 14:39:28+0200 [VumiRedis,client] Unhandled Error
        Traceback (most recent call last):
          File "/var/praekelt/vumi-go/ve/local/lib/python2.7/site-packages/twisted/internet/defer.py", line 304, in addCallback
            callbackKeywords=kw)
          File "/var/praekelt/vumi-go/ve/local/lib/python2.7/site-packages/twisted/internet/defer.py", line 293, in addCallbacks
            self._runCallbacks()
          File "/var/praekelt/vumi-go/ve/local/lib/python2.7/site-packages/twisted/internet/defer.py", line 575, in _runCallbacks
            current.result = callback(current.result, *args, **kw)
          File "/var/praekelt/vumi-go/ve/local/lib/python2.7/site-packages/twisted/internet/defer.py", line 1213, in unwindGenerator
            return _inlineCallbacks(None, gen, Deferred())
        --- <exception caught here> ---
          File "/var/praekelt/vumi-go/ve/local/lib/python2.7/site-packages/twisted/internet/defer.py", line 1070, in _inlineCallbacks
            result = g.send(result)
          File "/var/praekelt/vumi-go/go/vumitools/routing.py", line 769, in process_event
            "Could not find next hop for event.", event)
        go.vumitools.routing.UnroutableMessageError: ('Could not find next hop for event.', <Message payload="{'transport_name': u'go_loadtesting_transport', 'event_type': u'ack', 'event_id': u'46f9205a41f944dc8d5c6a590558337f', 'sent_message_id': u'bb6439a5eaa04f7c9a8eeacb0e51232f', 'helper_metadata': {u'go': {u'user_account': u'feab35c9260c46aca9c523ca7c6a3b29'}, u'tag': {u'tag': [u'go_loadtest', u'go-loadtest-01']}}, 'routing_metadata': {u'go_outbound_hops': [[[u'CONVERSATION:static_reply:43abd859b5014c4dbdf424ca712870cb', u'default'], [u'BILLING:OUTBOUND', u'default']], [[u'BILLING:OUTBOUND', u'default'], [u'TRANSPORT_TAG:go_loadtest:go-loadtest-01', u'default']]], u'go_hops': [[[u'TRANSPORT_TAG:go_loadtest:go-loadtest-01', u'default'], [u'BILLING:OUTBOUND', u'default']], [['BILLING:INBOUND', u'default'], None]], u'endpoint_name': u'default'}, 'message_version': u'20110921', 'timestamp': datetime.datetime(2014, 2, 18, 12, 39, 28, 465233), 'transport_metadata': {}, 'user_message_id': u'bb6439a5eaa04f7c9a8eeacb0e51232f', 'message_type': u'event'}">)
```

Note the discrepancy between the ``go_outbound_hops``:

```
[[['CONVERSATION:static_reply:43abd859b5014c4dbdf424ca712870cb', 'default'],
  ['BILLING:OUTBOUND', 'default']],
 [['BILLING:OUTBOUND', 'default'],
  ['TRANSPORT_TAG:go_loadtest:go-loadtest-01', 'default']]]
```

and the ``go_hops`` being built up for the event:

```
[[['TRANSPORT_TAG:go_loadtest:go-loadtest-01', 'default'],
  ['BILLING:OUTBOUND', 'default']],
 [['BILLING:INBOUND', 'default'], None]]
```