Satisfy rainbow dependency
Satisfy rainbow dependency

This is related to: https://github.com/sickill/rainbow/issues/44

I noticed while testing this branch that I could not build the docker
image without error:

```
docker build -t codeclimate/codeclimate-reek --no-cache .
```

This change fixes the issue. An alternative change would be to modify
Gemfile or reek.gemspec to promote rake from the development group.

troessner/reek,troessner/reek