title: Heroku command: Heroku Rake db:migrate fails
body: <p>When I run Heroku db:migrate I get an error:</p>
  
  <pre><code>rake aborted!
  no such file to load -- sqlite3
  /disk1/home/slugs/313735_a606978_6916/mnt/Rakefile:4
  (See full trace by running task with --trace)
  (in /disk1/home/slugs/313735_a606978_6916/mnt)
  </code></pre>
  
  <p>When I do </p>
  
  <pre><code>heroku rake db:push
  </code></pre>
  
  <p>I succeed.</p>
  
  <p>Do I need to install something on the Heroku server?</p>