Separate test dev dependencies

  Remove from gemspec development deps not required to run tests
    
    per discussion in https://github.com/troessner/reek/pull/226
    
    The gems moved into the Gemfile group :local_development may be
    used while developing the gem, but are not required for running tests.
    
    Now, one can run the tests without installing all the development gems with

```sh
      bundle --without local_development
      bundle exec rake
```

    The choice of 'local_development' has to do with how Bundler brings
    evaulates the gemspec development dependencies as both development
    and test dependencies.