Add MIDDLEWARE_CLASSES to runtests.py

Django 1.7 throws a warning:

```
?: (1_7.W001) MIDDLEWARE_CLASSES is not set.
HINT: Django 1.7 changed the global defaults for the MIDDLEWARE_CLASSES. django.contrib.sessions.middleware.SessionMiddleware, django.contrib.auth.middleware.AuthenticationMiddleware, and django.contrib.messages.middleware.MessageMiddleware were removed from the defaults. If your project needs these middleware then you should configure this setting.
```

Setting it empty on runtests.py by default might be helpful :)