Update contributors page's Continuous Integration section to remove Travis CI

Several months ago we had a very brief chat on the {{#zeppelin }}slack channel about the Contributors page:
https://zeppelin.apache.org/contribution/contributions.html

The section entitled Continuous Integration says that potential contributors to the project "...should setup travis-ci for the fork before making a pullrequest."

Jeff said in that thread that this section is outdated and Zeppelin uses github actions now.

This ticket is so we can keep track of the fact that the contributors page needs to be updated to reflect the fact that travis-ci is no longer necessary.