OJT21-52: add rubocop and modify ruby version in gemfile

 source 'https://rubygems.org'
 git_source(:github) { |repo| "https://github.com/#{repo}.git" }
 
-ruby '2.7.3'
+ruby '2.6.7'
 
 # Bundle edge Rails instead: gem 'rails', github: 'rails/rails', branch: 'main'
 gem 'rails', '~> 6.1.4'
@@ -34,6 +34,8 @@ group :development do
   gem 'listen', '~> 3.3'
   # Spring speeds up development by keeping your application running in the background. Read more: https://github.com/rails/spring
   gem 'spring'
+
+  gem 'rubocop', require: false
 end
 
 # Windows does not include zoneinfo files, so bundle the tzinfo-data gem