By using tox instead of the default Travis-CI Python environments, we
ensure that we have a single entrypoint to testing both locally and in
CI. This reduces redundant code and makes it clear when test
environments don't match up on different platforms.
[tox-travis](https://tox-travis.readthedocs.io/en/stable/) is introduced
here to automatically run tox jobs under the proper Travis-CI
environments. Additionally, the coveralls step is moved to a [build
stage](https://docs.travis-ci.com/user/build-stages) to run once after
all other Travis-CI tests complete.
