Update requests usage to match later requests.

```
File "/var/praekelt/vumi-go/ve/local/lib/python2.7/site-packages/django/views/generic/base.py", line 86, in dispatch
    return handler(request, *args, **kwargs)

  File "/var/praekelt/vumi-go/go/apps/dialogue/view_definition.py", line 40, in get
    model_data = r.json['result']['poll']

TypeError: 'instancemethod' object has no attribute '__getitem__'
```