There were a few changes needed:
* Plugins are now specified through `plugin:` config keyword.
* All plugin gems need to be specified explicitly in Gemfile since they
are no longer dependencies of plugins already specified explicitly.
* All plugin gems need to be updated in other to use the new APIs.
* One cop was renamed.
* New offenses safe to correct were corrected directly with `bundle exec
rubocop -a`.
* New offenses unsafe to correct were added to the TODO configuration
with `bundle exec rubocop --auto-gen-config --auto-gen-only-exclude
--exclude-limit 1400 --no-auto-gen-timestamp`.
