This PR changes the CI from using Travis to using Drone. the .drone.jsonnet file is a "shortened" configuration file from which you can generate the .drone.yml. Each PHP version is run in its own pipeline. Otherwise it is a pretty carbon copy of Travis. When this PR is accepted, we have to remove the Travis trigger from the repo. Each PHP pipeline waits 20 seconds for the RDBMSs to start up. I've also removed some remnants of Hound CI. Somehow I don't really know what else to write here...
Since this already removes the travis.yml, there wont be a successfull report from the Travis CI anymore.