Use --cov-append option in travis build to include coverage data from all tox environments run on the travis environment.

Note that it is run as one per python version, but multiple versions of django are tested in each.

.travis.yml

@@ -8,7 +8,7 @@ python:
 - 3.6
 install: pip install tox-travis codecov
 # positional args ({posargs}) to pass into tox.ini
-script: tox -- --cov
+script: tox -- --cov --cov-append
 after_success: codecov
 deploy:
   provider: pypi