title: fix `Missing frozen string literal comment` issue
body: <p>I created a new migration, it looks like this one:</p>
  
  <pre><code>class AddCommentsToUsers &lt; ActiveRecord::Migration
    def change
      add_column :users, :comments, :text
    end
  end
  </code></pre>
  
  <p>Now with Code Climate I am warned of an issue:
  <code>Missing frozen string literal comment.</code></p>
  
  <p>I tried to fix it like this:</p>
  
  <pre><code># frozen_string_literal: true
  class AddCommentsToUsers &lt; ActiveRecord::Migration
    def change
      add_column :users, :comments, :text
    end
  end
  </code></pre>
  
  <p><b>But I still have the same issue. How can I solve it?</b> Thanks.</p>