Travis CI automated testing

[alexbw]

We shouldn't need to worry if a commit breaks a test or not -- it should be run automatically. Once the pip install situation stabilizes (i.e., we do not want to build xlapy from scratch at every commit or PR), this should not be too difficult to do.

[vishwakftw]

I think it would be beneficial to add a Travis configuration that does a lint check (for e.g. flake8 or pep8), so that incoming PRs can follow the coding standards.