Changed get_query_set to get_queryset for compatibility with Django 1.7 ...

Fix for error when using Django 1.7 and later.
>Z:\metric\venv\lib\site-packages\periodically\models.py:6: RemovedInDjango18Warning: `BaseManager.get_query_set` is deprecated, use `get_queryset` instead.
>  qs = self.get_query_set().order_by('-start_time').all()