Switching from Travis to GitHub Actions

Hi all,

This is just a heads-up. I'll switch our build automation from Travis
to GitHub Actions soon. This is recomendation from Apache as Travis
announced end of unlimited OpenSource support [1]
But I'm personally prefer Actions for quite some time, so I think it's
a good thing.

I've already checked Cayenne builds [2] and it's much faster than on Travis.

[1] https://cwiki.apache.org/confluence/pages/viewpage.action?spaceKey=INFRA&title=Travis+Migrations
[2] https://github.com/apache/cayenne/actions/

-- 
Best regards,
Nikita Timofeev