title: Force Travis to build a specific pull request or commit SHA that it missed
body: <p>Is there a way to tell <a href="http://travis-ci.org" rel="noreferrer">travis</a> to build a specific SHA or the HEAD of a pull request that it might have previously missed (ie if the webhook failed to fire or get picked up by travis)?</p>
  
  <p>I could push a new commit to that pull request, but a method that doesn't involve changing the branch would be better, either via the travis UI, or travis' or github's APIs.</p>