Switch from Travis to another CI tool

The delays in Travis builds have grown to several hours, and on many cases Travis does not trigger at all as some Apache account limit is reached.

I raised an issue re this with the Apache Infra team. Their response was:

"Our Travis CI capacity is very limited and we will not be increasing that limit in the foreseeable future. If you are not happy with it, we suggest you switch to GitHub Actions or our Jenkins/BuildBot CI instead."

Given this, we should explore to change to Github Actions instead.