Add drone.io configuration

This PR adds a working .drone.yml configuration that will be used by our drone deployment.

Signed-off-by: Andrea Luzzardi <aluzzardi@gmail.com>
Oh, for future reference, this could just be `AUTO_GOPATH=1`, and then you could remove both the `mkdir` and `ln -s` lines below and `hack/make.sh` will do that for you automatically.