Pin RuboCop to 0.81.0 to support Ruby 2.3

  - This is to get to a green build
  - RuboCop 0.82.0 stopped supporting 2.3: https://github.com/rubocop-hq/rubocop/releases/tag/v0.82.0

Co-Authored-By: Chaymae Bazzaoui <bazzaoui.chaymae@gmail.com>

Gemfile

@@ -16,7 +16,7 @@ group :development do
 end
 
 group :test, :development do
-  gem 'rubocop', require: false
+  gem 'rubocop', '0.81.0', require: false # This version supports Ruby 2.3
 end
 
 group :guard do