DPK-51: Add a matrix of Django and python versions

.travis.yml

@@ -1,11 +1,20 @@
 language: python
 env:
     PYTHONPATH: .
+    matrix:
+      - DJANGO='>=3.1,<3.2'
+      - DJANGO='>=3.0,<3.1'
+      - DJANGO='>=2.2,<3.0'
+      - DJANGO='>=2.1,<2.2'
+      - DJANGO='>=2.0,<2.1'
 python:
-  - "3.6.8"
+  - 3.8
+  - 3.7
+  - 3.6
 install:
   - pip install -r requirements.txt
   - pip install coverage coveralls
+  - pip install --pre django$DJANGO
   - pip install .
 script:
   - coverage run --source=django_presskit runtests.py