title: foreigner - remove foreign key
body: <p>I am trying to use mailboxer in my rails 4 app. A problem is arising when i try to deploy the db. The error occurs in creating the mailboxer conversations table, which has dependencies in notifications table. </p>
  
  <p>I am trying to remove the foreign key for notifications conversations.</p>
  
  <p>I created a migration which says:</p>
  
  <pre><code>change_table :notifications do |t|
  t.remove_foreign_key :conversations
  </code></pre>
  
  <p>However, the rake aborts and says a foreign key does not exist.</p>
  
  <pre><code>rake aborted!
  An error has occurred, this and all later migrations canceled:
  
  PG::UndefinedObject: ERROR:  constraint "notifications_conversation_id_fk" of relation      "notifications" does not exist
  </code></pre>
  
  <p>My schema includes:
      add_foreign_key "notifications", "conversations", name: "notifications_on_conversation_id"</p>
  
  <p>I tried to rake db:migrate:down the original migration that created mailboxer, but also got an error saying 'command not found'. </p>
  
  <p>Can anyone help? Thank you.</p>