Suppress RuboCop's warnings

This PR suppresses the following RuboCop's warnings:

```console
rubocop/cop/internal_affairs extension supports plugin, specify `plugins: rubocop/cop/internal_affairs`
instead of `require: rubocop/cop/internal_affairs` in /Users/koic/src/github.com/rubocop/rubocop-ast/.rubocop.yml.
For more information, see https://docs.rubocop.org/rubocop/plugin_migration_guide.html.

Specify `rubocop-internal_affairs` instead of `rubocop/cop/internal_affairs` in your configuration.

rubocop-performance extension supports plugin, specify `plugins: rubocop-performance`
instead of `require: rubocop-performance` in /Users/koic/src/github.com/rubocop/rubocop-ast/.rubocop.yml.
For more information, see https://docs.rubocop.org/rubocop/plugin_migration_guide.html.
```

Author: koic

.rubocop.yml

@@ -1,9 +1,12 @@
 # This is the configuration used to check the rubocop source code.
 
 inherit_from: .rubocop_todo.yml
-require:
-  - rubocop/cop/internal_affairs
+
+plugins:
+  - rubocop-internal_affairs
   - rubocop-performance
+
+require:
   - rubocop-rspec
 
 AllCops: