Travis no longer provides free CI for open source packages, see https://discourse.julialang.org/t/travis-ci-changes-pricing-plans/49528

I have tried to apply for open source credits, but depending on if they approve it and how often we will have to re-apply, it might not be a great long-time solution, so we consider changing to github actions which is still free for open source projects.
If someone wants to try with a PR, there are some code snippets that can probably be used here
https://discourse.julialang.org/t/easy-workflow-file-for-setting-up-github-actions-ci-for-your-julia-package/49765

Edited: I have applied for credits