Patch RuboCop::Config to warn on obsolete cops

Instead of raising on the inclusion of an obsolete cop, write an error
to STDERR. This is slightly more invisible than failing a build on an
obsolete cop, but preferable as a user won't be blocked when we bump
RuboCop on our engine. We'll write a changelog post advising users of
the necessary changes to make and deprecate this patch in the coming
weeks.

Author: jpignata

lib/cc/engine/rubocop.rb

@@ -2,6 +2,7 @@
 require "delegate"
 require "pathname"
 require "rubocop"
+require "rubocop/config_patch"
 require "rubocop/cop/method_complexity_patch"
 require "rubocop/cop/method_length"
 require "rubocop/cop/class_length"

lib/rubocop/config_patch.rb

@@ -0,0 +1,10 @@
+RuboCop::Config.class_eval do
+  def reject_obsolete_cops
+    RuboCop::Config::OBSOLETE_COPS.each do |cop_name, message|
+      next unless key?(cop_name) || key?(cop_name.split('/').last)
+      message += "\n(obsolete configuration found in #{loaded_path}, please" \
+        ' update it)'
+      $stderr.puts message
+    end
+  end
+end