Add eslint-3 channel to engines manifest (#488)
Add eslint-3 channel to engines manifest (#488)

This commit configures an eslint-3 channel for the codeclimate-eslint
engine, wrapping [ESLint 3][]:

[ESLint 3]: http://eslint.org/blog/2016/07/eslint-v3.0.0-released

```yaml
engines:
  eslint:
    enabled: true
    channel: "eslint-3"
```
codeclimate/codeclimate,codeclimate/codeclimate,codeclimate/codeclimate