Export of events registrations is broken

Ghost User
2017-12-10T21:29:12+01:00

HTTP 500

```
Traceback:

File "/usr/local/lib/python3.5/site-packages/django/core/handlers/exception.py" in inner
41. response = get_response(request)

File "/usr/local/lib/python3.5/site-packages/django/core/handlers/base.py" in _get_response
187. response = self.process_exception_by_middleware(e, request)

File "/usr/local/lib/python3.5/site-packages/django/core/handlers/base.py" in _get_response
185. response = wrapped_callback(request, *callback_args, **callback_kwargs)

File "/usr/local/lib/python3.5/site-packages/django/contrib/auth/decorators.py" in _wrapped_view
23. return view_func(request, *args, **kwargs)

File "/usr/local/lib/python3.5/site-packages/django/contrib/auth/decorators.py" in _wrapped_view
23. return view_func(request, *args, **kwargs)

File "./events/views.py" in export
123. data['paid'] = _('Yes') if registration.paid else ''

Exception Type: AttributeError at /events/admin/212/export/
Exception Value: 'Registration' object has no attribute 'paid'
```

### Expected behaviour

No errors

### Steps to reproduce

1. Create an event
2. Add registrations
3. Export all registrations

### Possible solution

Fix the export to use the new `payment` field instead of `paid`.