Update Rubocop to 0.25.0 and address TODO in .rubocop.yml

As in the title. I went ahead and put all of the cops in their correct namespaces and organized them as well.

The Metrics/PerceivedComplexity is currently set at a max of 10, as that would allow all current code to pass without changes. The default is 7, so we're pretty close as it is!