CONTRIBUTING.md leads user to install ancient version of drone CLI

[stevegt]

Right now we're sending people to http://readme.drone.io/usage/getting-started-cli/, which installs drone CLI version 0.5.0. We instead might want to send people to http://docs.drone.io/cli-installation/, which seems to be better maintained (currently installs verson 0.8.5).

For background see conversation at techknowlogick#1 (comment).