Add rubocop-rake extension

This commit adds the rubocop-rake extension to Puppet's Gemfile and
Rubocop configuration file, pinning it at 0.6.0 as it's the most recent
version, and still supports Ruby 2.5 (which the rest of our Rubocop
configuration is set up to support).

Author: mhashizume

.rubocop.yml

@@ -3,6 +3,7 @@ inherit_from: .rubocop_todo.yml
 require:
   - rubocop-i18n
   - rubocop-performance
+  - rubocop-rake
 
 AllCops:
   TargetRubyVersion: 2.5

Gemfile

@@ -50,6 +50,7 @@ group(:test) do
   gem 'rubocop', '1.28.0', require: false, platforms: [:ruby]
   gem 'rubocop-i18n', '~> 3.0', require: false, platforms: [:ruby]
   gem 'rubocop-performance', '1.13.3', require: false, platforms: [:ruby]
+  gem 'rubocop-rake', '0.6.0', require: false, platforms: [:ruby]
 end
 
 group(:development, optional: true) do