Prefer reduce to inject

This is more inline with community conventions and is the new thoughtbot 
preference.

https://github.com/thoughtbot/guides/pull/237
This is how it's indented in the non-thoughtbot example, but it makes more sense to me to indent everything.

https://github.com/thoughtbot/hound/blob/jf-prefer-reduce-to-inject/spec/models/style_guide/ruby_spec.rb#L149