Move rubocop-rails and rubocop-rspec to plugins

New versions (#703 and #705) of these gems implement the new "plugin"
system for Rubocop. Having them as `require`s, still works, but emits a
warning (Which our Overcommit config says is an error).

.rubocop.yml

@@ -1,6 +1,8 @@
-require:
+plugins:
   - rubocop-rails
   - rubocop-rspec
+
+require:
   - rubocop-rspec_rails
   - rubocop-capybara
   - rubocop-factory_bot