Move to GitHub actions

[jneu]

This pull request moves the automated builds and installs from Travis CI to Github Actions. All attempts were made to duplicate the functionality exactly, but there is a one significant difference:

• Travis CI uses the Ubuntu 16.04 "xenial" base image, which is not available in Github Actions. Instead it uses Ubuntu 20.04, and the update doesn't appear to cause any issues during my testing.

Note once the PR is merged, a Github Secret will need to be created, called CODECOV_TOKEN, whose value is the codecov token in order for codecov uploads to work correctly.

Please let me know if you have any questions or concerns.