Update django-configurations to 2.1

[pyup-bot]

This PR updates django-configurations from 2.0 to 2.1.

Changelog

2.1

^^^^^^^^^^^^^^^^^

- **BACKWARD INCOMPATIBLE** Drop support of Python 3.3.

- **BACKWARD INCOMPATIBLE** Drop support of Django 1.9.

- Add support for Django 2.1.

- Add ``PositiveIntegerValue`` configuration value.

- Fix ``bool(BooleanValue)`` to behave as one would expect (e.g.
``bool(BooleanValue(False))`` returns ``False``).

- Miscellaneous documentation improvements and bug fixes.

Links

• PyPI: https://pypi.org/project/django-configurations
• Changelog: https://pyup.io/changelogs/django-configurations/
• Docs: https://django-configurations.readthedocs.io/

[pyup-bot]

Closing this in favor of #790