tox: test against django 2.2

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 toxworkdir = {env:TMPDIR:/tmp}/tox-{env:USER}/combo/{env:BRANCH_NAME:}
-envlist = coverage-py3-pylint
+envlist = coverage-django111-pylint,django22
 
 [testenv]
 usedevelop = True
@@ -11,7 +11,8 @@ setenv =
   TOX_WORK_DIR={toxworkdir}
   coverage: COVERAGE=--cov-report xml --cov-report html --cov=combo/
 deps =
-  django>=1.11,<1.12
+  django111: django>=1.11,<1.12
+  django22: django>=2.2,<2.3
   pywebpush
   pytest-cov
   pytest-django