Remove TravisCI integration from Apache Knox

We have to remove Travis CI integration based on https://cwiki.apache.org/confluence/pages/viewpage.action?spaceKey=INFRA&title=Travis+Migrations

Fortunately, we already have GitHub Actions in place with one exception: we need to figure out how to add Linux ARM64 support there (which we have with Travis).

As a pre-requisite, I merged a change 3 weeks ago that runs shellcheck as part of our builds: https://github.com/apache/knox/pull/646