Use GitHub Actions to manage Pushy releases

[jchambers]

Right now, the deployment process for Pushy is that I type a bunch of stuff into a terminal to manually trigger deployment to Maven Central using the Maven deploy plugin. This is more error-prone than is ideal, and also perhaps excessively dependent on my specific credentials.

As an alternative, we should consider using the Travis CI deployment system to build, sign, and publish artifacts at release time.

[jchambers]

Now that GitHub Actions are A Thing, we might consider using those, too.