Tweak coveralls configuration

coveralls.io is throwing 422 and breaking CI,
this may or may not help.

Related: https://github.com/TheKevJames/coveralls-python/issues/252

.github/workflows/django.yml

@@ -50,6 +50,6 @@ jobs:
         coverage run --omit=*/tests/* --source=hc manage.py test
     - name: Coveralls
       if: matrix.db == 'postgres' && matrix.python-version == '3.8'
-      run: coveralls
+      run: coveralls --service=github
       env:
         GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}