Fix RemovedInDjango30Warning from Django 2.0+

[adamchainz]

The latest pytest surfaces all deprecation warnings at the end of the test run, so I'm seeing this one hundreds of times on my app.

.../python3.6/site-packages/django/db/models/sql/compiler.py:997: RemovedInDjango30Warning: Remove the context parameter from HashidAutoField.from_db_value(). Support for it will be removed in Django 3.0.
  RemovedInDjango30Warning,

Should be an easy fix with conditional versions of the function for django versions.

[adamchainz]

This also implies the logic in runtests.py isn't working: https://github.com/nshafer/django-hashid-field/blob/master/runtests.py#L11

[nshafer]

How lost we are when our tools break. Thank you for the PR. Released in 2.1.4.

[adamchainz]

How philosophical! Thanks very much for the fast turnaround.

[adamchainz]

P.S. version 2.1.3 never existed on PyPI: https://pypi.org/project/django-hashid-field/#history 🤔

[nshafer]

That is weird... I'm pretty sure I did a publish since I created the 2.1.3 tag in git, and that happens after a publish as I copy and paste the lines output by python setup.py publish. Perhaps there was an error from twine that I missed or something. Since it was a fix specifically for installing from github instead of pypi, I guess it doesn't matter at all. Still, I'll make sure I check pypi after a publish. Thanks.