Add a basic GitHub Actions CI action that copies ".travis.yml".

Fixes #243.

* Replace *.github.com urls with *.github.io

* Fix two additional URL's having *.github.com

The SVG button's image had never been updated to display the
actual master-branch build status! Now it's fixed.
Also, use https:// URLs.

And add a TravisCI badge to the README.
https://travis-ci.org/troydhanson/uthash