👷 Add GitHub Actions based CI

[daenney]

• Add a GitHub Actions based CI pipeline
• Remove Travis-CI

[daenney]

@bonan Transport tests are failing with:

 # lib.hemtjan.st/transport/mqtt [lib.hemtjan.st/transport/mqtt.test]
transport/mqtt/message_test.go:32:11: cl.init undefined (type *mqtt has no field or method init)
transport/mqtt/mqtt_test.go:65:11: cl.init undefined (type *mqtt has no field or method init)
transport/mqtt/mqtt_test.go:105:11: cl.init undefined (type *mqtt has no field or method init)
transport/mqtt/mqtt_test.go:121:11: cl.init undefined (type *mqtt has no field or method init)
transport/mqtt/mqtt_test.go:140:11: cl.init undefined (type *mqtt has no field or method init)

What do I need to do to fix those?