Travis: Clean up .travis.yml
Travis: Clean up .travis.yml

- Add / update the environment matrix to be more maintainable.
- Upgrade to Xenial
- Test on Python 3.7
- Test on Postgres 10
- Remove sudo: false =>
  https://blog.travis-ci.com/2018-11-19-required-linux-infrastructure-migration

dj-stripe/dj-stripe,dj-stripe/dj-stripe,kavdev/dj-stripe,pydanny/dj-stripe,kavdev/dj-stripe,pydanny/dj-stripe