[postgis-devel] GH Actions

[Paul Ramsey]

Since Travis is now basically inoperative, I have added a working GitHub Actions configuration to the repository, and stripped out all the travis badges.

Seems like you don't need anything special to get GHA running in your personal repo, so you can get easy CI testing on your working branches just by pushing to your working fork. #magic

All hail Microsoft, source of all that is good and light.

P.