Add drone.yml and remove travis

[jvican]

Travis, goodbye. Your last attempt of killing our beloved tut setup is
not nice. I'm sorry, this is how life is. We just change partner.

See urho3d/urho3d#651 for related discussions
on memory leaks in Travis and Travis killing other processes that are
memory intensive as tut.

[heathermiller]

I'm confused by the order of operations here. Why is there drone output about failed CI before this PR has been merged?

[jvican]

Travis hasn't kicked out because it doesn't find a travis.yml file, and Drone has already this repository enabled (because I did enable it in the admin panel). If it finds drone.yml it will try to build it. I'm now choosing the correct image that has both java and the correct ruby version, and this will be good to go.

[heathermiller]

There's no Travis output in this PR's checks. Only broken drone output, claiming that coursier can't be found.

[jvican]

Yes, there's no Travis output because Travis doesn't kick out any if there's no travis.yml file, and this commit removes it.

I'm working on fixing the drone output.

[jvican]

@heathermiller This is finally ready for review, everything passes in Drone. I took the liberty to remove the coursier installation from the drone script and add coursier directly to the image we depend on.

As a data point, Drone builds everything in ~5 minutes where Travis did it in around ~10 (this includes the optimization that we merged some hours ago).

[heathermiller]

LGTM, thanks @jvican!! 🎉