don't use lambda for default value of uuid

Seems to break migrations in django 1.7, not sure why..

```
./manage.py makemigrations djmail

...
File "/home/thijs/.virtualenvs/foo/local/lib/python2.7/site-packages/django/db/migrations/writer.py", line 337, in serialize
    raise ValueError("Cannot serialize function: lambda")
ValueError: Cannot serialize function: lambda
```