Convert from TravisCI to GithubActions

[kmanning]

TravisCI is moving away from supporting free plans. Let's convert from TravisCI to GithubActions

• Run the gradle task to execute tests
• Replace the (now-deprecated) CodeCov BashUploader with a native GithubAction
• Remove the old travisCI and codecov BashUploader
• Update the README BuildBadge to use Github instead of TravisCI

[kmanning]

This coincidentally solves Issue #394 - we can use a GithubAction to upload Codecov metrics, in place of bash uploader.