- Migrate `.codeclimate.yml` to version 2 structure
- Disable Code Climate [maintainability checks][]

https://docs.codeclimate.com/docs/advanced-configuration

[maintainability checks]: http://blog.codeclimate.com/blog/2017/10/12/10-point-technical-debt-assessment

This commit upgrades the Code Climate configuration file and disables
the new maintainability checks.

Once this PR is merged, we can experiment with the new checks via pull
requests (the PR will report new issues) or via the [Code Climate
CLI][cli], which [now also supports the new maintainability
checks][cli-support].

[cli]: https://github.com/codeclimate/codeclimate
[cli-support]: https://codeclimate.com/changelog/5a0e488824cfa902a300091c