Convert CI from Travis to Github Actions

[ar-jan]

Changing to Github Actions should simplify things (fewer external services), and doesn't have a limit on free testing capacity.

[ar-jan]

It looks like the way to use postgres in an Action is via a service container. This uses the Docker postgres image.

I think this means we can't rely on peer authentication, and instead have to provide a TCP port-based connection string witih password. Thoughts?

[jgonggrijp]

I think you are right.

[ar-jan]

Done.