Update djangorestframework-gis to 0.18

[pyup-bot]

This PR updates djangorestframework-gis from 0.15 to 0.18.

Changelog

0.17.0

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

- [feature] Added
`OpenAPI Schema Generation <https://github.com/openwisp/django-rest-framework-gis#schema-generation>`_
(`219 <https://github.com/openwisp/django-rest-framework-gis/issues/219>`_) - special
thanks to `Dhaval Mehta <https://github.com/dhaval-mehta>`_

0.16.0

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

- [fix] Added support for representation of empty geometries
(`168 <https://github.com/openwisp/django-rest-framework-gis/issues/168>`_)
- [fix] Don't override the additional arguments passed as ``style`` to ``GeometryField``
- [feature] Added `DistanceToPointOrderingFilter <https://github.com/openwisp/django-rest-framework-gis#distancetopointorderingfilter>`_
(`210 <https://github.com/openwisp/django-rest-framework-gis/issues/210>`_)
- [deps] Added support for django 3.1 in the CI build
- [deps] Dropped django 1.11 and Python 3.5 from the CI build,
compatibility may still work but it's not guaranteed anymore, please upgrade!
- [qa] Added QA checks to CI build
(`230 <https://github.com/openwisp/django-rest-framework-gis/issues/230>`_)

Links

• PyPI: https://pypi.org/project/djangorestframework-gis
• Changelog: https://pyup.io/changelogs/djangorestframework-gis/
• Repo: https://github.com/openwisp/django-rest-framework-gis/releases

[pyup-bot]

Closing this in favor of #288