Django >= 1.7 compatibility for requires_system_checks

Hi @dcramer,

Devserver will fail to start up on Django 1.7 with the error:

```
django.core.exceptions.ImproperlyConfigured: Command Command defines both "requires_model_validation" and "requires_system_checks", which is illegal. Use only "requires_system_checks".
``` 

The django-devserver runserver management command inherits from the django core runserver management command which set `requires_system_checks = False`. The django-devserver runserver management command then sets `requires_model_validation = False`.

This pull request ensures only the appropriate attribute is set to False, so is backwards compatible with Django < 1.7.

The implementation is only suggested. If you have another way you'd prefer to do it that's fine, this was just a convenient way to illustrate the issue.