Support Django 2.2

[Moliholy]

Description: Describe in a couple of sentence what this PR adds This PR adds support for Django 2.2 LTS

Dependencies: dependencies on other outstanding PRs, issues, etc.

• https://github.com/vintasoftware/drf-rw-serializers/issues/14

Merge deadline: List merge deadline (if any) None.

Installation instructions: List any non-trivial installation instructions. None.

Testing instructions: None.

Reviewers:

• [ ] tag reviewer
• [ ] tag reviewer

Merge checklist:

• [ ] All reviewers approved
• [ ] CI build is green
• [ ] Version bumped
• [ ] Changelog record added
• [ ] Documentation updated (not only docstrings)
• [ ] Commits are squashed
• [ ] PR author is listed in AUTHORS

Post merge:

• [ ] Create a tag
• [ ] Check new version is pushed to PyPi after tag-triggered build is finished.
• [ ] Delete working branch (if not needed anymore)

Author concerns: List any concerns about this PR - inelegant solutions, hacks, quick-and-dirty implementations, concerns about migrations, etc.

[hugobessa]

Hi @Moliholy, I thanks for your PR. I reviewed and it seems good to go. I'll merge and release by next weekend ok?

[Moliholy]

Hi, @hugobessa! That sounds pretty good, thanks a lot!