After travis-ci got bought and layed off a lot of key staff I did think the day would come where they will no longer will be the defacto choice for open source and I think this news confirms that day being today :-)
OSS projects are eligible for a free Gold plan on GitLab.com and can benefit from more CI minutes: https://about.gitlab.com/solutions/open-source/
If you are planning to continue using GitHub, you can also integrate with GitLab CI/CD for your OSS project: https://about.gitlab.com/solutions/github/
EDIT: Travis seems to take 10 credits per minute, with 1000 free credits per month. So it seems to be the least generous tool with 100 minutes per month vs 400 (GitLab CI) and 2000 (GitHub Actions).
> You have 1,000 credits left - these will begin counting down automatically as soon as you run your first build.
> You can use your credits to build on both private and open-source repositories using Linux, macOS, and Windows OS.
> 1,000 credits will be replenished automatically monthly. Additional Credits purchase is not available for Free Plan.
Even if the email is accurate, which is the better scenario, though, 1000 credits is 100 minutes of Linux build time (or 50 on Windows, 20 on Mac). Which is not much for any but the smallest projects.
I find this pay it forward model better than any other option.