title: rubocop how do you fix Missing magic comment
body: <p>I have a library of ruby code, and to look for defects I run</p>
  
  <pre><code>$ rubocop
  </code></pre>
  
  <p>And I get</p>
  
  <pre><code>$ rubocop
  Inspecting 153 files
  CCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCWCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCWCCCCCCCWWCCCCC
  
  Offenses:
  
  Gemfile:1:1: C: Missing magic comment # frozen_string_literal: true.
  source "https://rubygems.org"
  </code></pre>
  
  <p>What modifications are required in my Gemfile to make rubocop not complain?</p>