Add support for open-sourced DroneCI

[teohm]

Open-sourced DroneCI uses a different set of environment variables:
http://readme.drone.io/usage/environment-reference/

This PR adds open-sourced DroneCI support.

We're using a self-hosted DroneCI, and just tested this PR. So far it's working.

Note: Before submit this PR, we also contacted CodeClimate Support. I think our support ticket is still opened, in case you need more context of this PR. 🙏

[dblandin]

Hey @teohm!

Thanks for the PR! I encourage you to check out our new test reporter, which is currently in public beta.

You should be able to add DroneCI env var support by updating these two files:

https://github.com/codeclimate/test-reporter/blob/master/env/ci.go#L30
https://github.com/codeclimate/test-reporter/blob/master/env/git.go#L159

The new test reporter is faster, distributed as a static binary, has support for parallelized CI builds, and will receive ongoing support by the team here. The existing test reporters for ruby, python, php, and javascript are not yet deprecated but they're unofficially in maintenance mode.

Let me know if you have any questions. Thanks!

cc/ @davehenton

[dblandin]

Going to go ahead and close this in favor of codeclimate/test-reporter#215

Thanks @teohm!