Use GitHub Actions for CI

Use GitHub Actions for CI

Use Github Actions instead of Travis and AppVeyor.

In the Windows installer config, add support for Visual Studio 2019
(aka 16.0).

[ghudson@mit.edu: switched to Ubuntu 18.04 for Linux builds; removed
macOS build job for now; added more packages to avoid skipping tests;
made it easier to see skipped tests and to see files not cleaned;
added make install command; adjusted Windows build path]