Update pylint-django to 2.2.0

[pyup-bot]

This PR updates pylint-django from 2.1.0 to 2.2.0.

Changelog

2.2.0

---------------------------

- Rely on Django to resolve string references in ForeignKey fields. Refs
`243 <https://github.com/PyCQA/pylint-django/issues/243>`_ (Alejandro Angulo)
- Suppress ``unused-argument`` for functions in migration modules. Fix
`267 <https://github.com/PyCQA/pylint-django/issues/267>`_
- New checker for hard-coded ``auth.User``. Fix
`244 <https://github.com/PyCQA/pylint-django/issues/244>`_

Links

• PyPI: https://pypi.org/project/pylint-django
• Changelog: https://pyup.io/changelogs/pylint-django/
• Repo: https://github.com/PyCQA/pylint-django