Remove Travis CI builds or provide GitHub API token

[joschi]

The build jobs on Travis CI for macOS are currently failing because the workers have exhausted the GitHub API rate-limit.

Example: https://travis-ci.org/halcyon/asdf-java/jobs/656934756#L246-L279

The GitHub Actions workflow doesn't have this problem because they can use an automatically generated GitHub API token: https://help.github.com/en/actions/configuring-and-managing-workflows/authenticating-with-the-github_token

In order to allow for a successful build status for commits and PRs, either remove the Travis CI builds completely in favor of the existing GitHub Actions workflow or provide a valid GitHub API token for builds.

[halcyon]

I should be able to add a token in the near future to resolve this.