Add missing envlist for Django 2.2 support

[sliverc]

Missing change in #640

[n2ygk]

LGTM (again:-)

[codecov]

Codecov Report

Merging #641 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #641   +/-   ##
=======================================
  Coverage   96.16%   96.16%           
=======================================
  Files          56       56           
  Lines        2866     2866           
=======================================
  Hits         2756     2756           
  Misses        110      110

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[n2ygk]

@sliverc weirdness on the home front again. Travis build went just fine but when I do tox -e py37-df20-django22-drf39 all the test_filters fail:

E                   django.db.utils.IntegrityError: The row in table 'example_entry_authors' with primary key '1' has an invalid foreign key: example_entry_authors.author_id contains a value '1' that does not have a corresponding value in example_author.id.

.tox/py37-df20-django22-drf39/lib/python3.7/site-packages/django/db/backends/sqlite3/base.py:318: IntegrityError

I'm running a full tox now to see if this happens for other versions. So far on older versions there are no complaints. I wonder what other environment/cached thing is causing this...

[sliverc]

The blogentry.json has invalid authors referenced. This shouldn't have worked before either... yeah well PR is coming up.

[sliverc]

It is properly a difference in sqlite versions (where SQLite Version on Travis is much older). PR #642 fixing this.