Definindo um valor padrão para Booleano

Para evitar etse aviso quando executar a aplicação:

System check identified some issues:

WARNINGS:
pagseguro.Checkout.success: (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.