Fix poll loading that broke after migration to sandbox toolkit v2.

The change needed is:

```diff
diff --git a/go/apps/dialogue/vumi_app.js b/go/apps/dialogue/vumi_app.js
index 63c6ad3..f6c1295 100644
--- a/go/apps/dialogue/vumi_app.js
+++ b/go/apps/dialogue/vumi_app.js
@@ -32,10 +32,10 @@ var DialogueApp = App.extend(function(self) {
 
     self.get_poll = function() {
         return self
-            .im.sandbox_config.get('poll', {json: true})
+            .im.sandbox_config.get('poll', {json: false})
             .then(function(poll) {
                 return _.defaults(poll, self.poll_defaults);
-           });
+            });
     };
 
     self.add_state = function(desc) {
```

but we also need to fix tests.