Fixed two warnings in Django 1.7

Fixed the following deprecation warning:

    django_fsm_log/managers.py:18: RemovedInDjango18Warning: `StateLogManager.get_query_set` method should be renamed `get_queryset`.
        class StateLogManager(models.Manager):

See https://code.djangoproject.com/ticket/15363

I also added empty MIDDLEWARE_CLASSES to settings in runtests to fix a system check warning when running unit tests:
    System check identified some issues:

    WARNINGS:
    ?: (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.