Migrate to GH Actions ?

[jrfnl]

I'm looking to create a PR to address Yoast/PHPUnit-Polyfills#62 and just noticed that the CI is still on travis-ci.org, which (for the most part) is no longer functional.

@stof Would it be okay if I migrate the CI checks to GitHub Actions to get them running again ?

[stof]

@jrfnl yes

[jrfnl]

PR #35 has been opened to address this.