Tox: Use older pytest-django for Django 1.7
Tox: Use older pytest-django for Django 1.7

It seems that pytest-django 3.2.1 doesn't work with Django 1.7, so use
older version of pytest-django (3.1.x) with Django 1.7.

The build failed with an error with the following traceback with
pytest-django 3.2.1 and Django 1.7:

    File ".../pytest_django/plugin.py", line 153, in _setup_django
      _blocking_manager.block()
    File ".../pytest_django/plugin.py", line 641, in block
      self._save_active_wrapper()
    File ".../pytest_django/plugin.py", line 624, in _save_active_wrapper
      return self._history.append(self._dj_db_wrapper.ensure_connection)
    File ".../pytest_django/plugin.py", line 614, in _dj_db_wrapper
      from django.db.backends.base.base import BaseDatabaseWrapper

    ImportError: No module named base.base

kcsry/django-form-designer,andersinno/django-form-designer,kcsry/django-form-designer,andersinno/django-form-designer,andersinno/django-form-designer-ai,andersinno/django-form-designer-ai