Maintenance of documentation and tests

[smmaurer]

Some of our third-party workflow integrations have become glitchy, and this PR replaces them with native GitHub functionality. No substantive changes to code.

• Moves the doc building workflow off of ReadTheDocs. The new workflow is to build the docs manually and host them from the gh-pages branch. This is more reliable and makes troubleshooting easier; it's what we're doing across the UDST repos. See instructions in docs/README.md

• Moves the automated tests from Travis to GitHub Actions. We tried this out in https://github.com/UDST/pandana/pull/153 and it's working great.

  GitHub Actions runs more quickly and reliably, and supports cross-platform testing better. It also lets us write separate scripts for different tasks, for example running a quick set of tests each time code is pushed but specifying a longer list of python versions and operating systems when a PR is opened.

• Replaces Coveralls integration (for reporting test coverage) with a GitHub Actions script + bot

[github-actions[bot]]

Test coverage is 77%