changes to continuous integration

[juliaferraioli]

We will be dropping our paid Travis CI plan at the end of 2021. We do not expect there to be any visible changes to this repo, but wanted to give some notice just in case. We recommend migrating CI jobs to GitHub Actions.

Travis CI provides free testing for open source projects. In addition, Twitter has paid for a small number of additional concurrent builds which were available for open source as well as private repositories. Many Twitter projects have already moved to GitHub Actions for CI, and we have no private repos left using Travis, so we will be discontinuing our plan at the end of 2021.

Since this repo is open source, we do not expect this change to impact Travis CI builds for this project. However, we still recommend most Twitter projects to migrate to GitHub Actions for CI at your convenience.