Update changuito to use Django 3.0

[angvp]

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There's a branch with initial work where tests pass feature/upgrade-django3.0 but need to be tested .. and more unit tests should be added

• [x] Upgrade Travis config
• [x] Upgrade tox config
• [ ] Add more tests to proxy.py
• [ ] Add more tests to the middleware
• [ ] Add tests to signal actually 0% coverage
• [ ] Doing some functional test of the app
• [ ] Create a sample project and test it works

For checking coverage: https://coveralls.io/github/angvp/django-changuito?branch=feature/upgrade-django3.0