title: add_foreign_key not creating foreign keys
body: <p>Im using Rails 5.1 and SQLite. The below migration is not working as expected.</p>
  
  <pre><code>class AddJobTitleForeignKeyToTimeOffTypes &lt; ActiveRecord::Migration[5.1]
    def change
      add_column :time_off_types, :job_title_id, :integer
      add_foreign_key :time_off_types, :job_title, :column =&gt; :job_title_id
    end
  end
  </code></pre>
  
  <p>It creates the column "job_title_id" in the table "time_off_types" but it does not create the foreign key.</p>