travis-ci.com builds stalled: switch to GitHub Actions

[n2ygk]

@sliverc Travis-CI.com reportsBuilds have been temporarily disabled for public repositories due to a negative credit balance. Please go to the Plan page to replenish your credit balance or alter your Consume paid credits for OSS setting.

When I try to go to the Plan page it throws an error. See also this article: https://news.ycombinator.com/item?id=25338983

Looks like it's time to switch to GitHub actions.

[n2ygk]

@sliverc Let me know if you'd like me to take a crack at configuring this.