Lock in JSONField version.

JSONField 0.9.4 produces errors when migrating, eg:

```
Error in migration: actstream:0006_auto__add_field_action_data
 - Sending post_syncdb signal for actstream: ['Action', 'Follow']
Adding permission 'actstream | follow | Can add follow'
Adding permission 'actstream | follow | Can change follow'
Adding permission 'actstream | follow | Can delete follow'
Adding permission 'actstream | action | Can add action'
Adding permission 'actstream | action | Can change action'
Adding permission 'actstream | action | Can delete action'
ValueError: No JSON object could be decoded
```

Locking in the version at 0.9.3 removes the errors.