models: add default value to Occurrence.cancelled.

Avoid the following warning:
```
System check identified some issues:

WARNINGS:
calendarium.Occurrence.cancelled: (1_6.W002) BooleanField does not have a default value.
        HINT: Django 1.6 changed the default value of BooleanField from False to None. See https://docs.djangoproject.com/en/1.6/ref/models/fields/#booleanfield for more information.
```