Eliminate unnecessary jobs from Travis CI

[seisman]

We already have full tests (Python 3.6-3.8 on Linux, macOS and Windows)
running on GitHub Actions. No need to waste CI resources on Travis.

Address #566.

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If adding new functionality, add an example to docstrings or tutorials.

[weiji14]

Looks fine. I've also disabled the 'continuous-integration/travis-ci' required check since it won't be needed anymore.