Use GitHub Actions for CI integration

[glensc]

This drops Travis CI integration in favor of GitHub Actions.

🎩 🎉 thank you for being out there for that long (initial commit from 2013: af747d128d0478c7306a91cf17d701e47b538c87).

refs:

• https://github.com/shivammathur/setup-php
• https://help.github.com/en/actions/automating-your-workflow-with-github-actions/workflow-syntax-for-github-actions#jobsjob_idservices
• https://docs.github.com/en/free-pro-team@latest/actions/guides/creating-postgresql-service-containers
• https://firefart.at/post/using-mysql-service-with-github-actions/

[glensc]

mongodb example:

• https://github.com/actions/example-services/pull/21/files

[glensc]

@markstory please remove travis hook or service.

there's no point sending events to travis after each push:

• https://travis-ci.org/github/perftools/xhgui/requests

[markstory]

@glensc Done. I removed the webhook configuration.