title: Rails: how to create a migration that add 'using: :btree' to foreign key
body: <p>I have two models<br>
  father has_many sons<br>
  son belongs_to father</p>
  
  <p>Normally, this piece of code</p>
  
  <pre><code>def change
      add_reference :sons, :father, index: true
  end
  </code></pre>
  
  <p>it will generate</p>
  
  <pre><code>add_index "sons", ["father_id"], name: "index_sons_on_father_id"
  </code></pre>
  
  <p>Now, I want it generate </p>
  
  <pre><code>add_index "sons", ["father_id"], name: "index_sons_on_father_id", using: :btree
  </code></pre>
  
  <p>How to write the migration?</p>