Use RuboCop RSpec 3.5 for development

This commit uses RuboCop RSpec 3.5 for development and suppresses the following warning:

```console
$ bundle exec rubocop
rubocop-rspec extension supports plugin, specify `plugins: rubocop-rspec` instead of
`require: rubocop-rspec` 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

@@ -5,8 +5,6 @@ inherit_from: .rubocop_todo.yml
 plugins:
   - rubocop-internal_affairs
   - rubocop-performance
-
-require:
   - rubocop-rspec
 
 AllCops:

Gemfile

@@ -30,7 +30,7 @@ else
     gem 'rubocop', '>= 1.0'
   end
   gem 'rubocop-performance'
-  gem 'rubocop-rspec', '~> 3.0.0'
+  gem 'rubocop-rspec', '~> 3.5.0'
 end
 
 local_gemfile = File.expand_path('Gemfile.local', __dir__)