title: How to fix a permission denied (publickey) error for a git submodule update in the Github Travis CI build?
body: <p>I cannot update the git submodule, with the error:</p>
  
  <pre><code>$ git submodule init
  Submodule 'build/html' (git@github.com:quadroid/clonejs.git) registered for path 'build/html'
  ...
  $ git submodule update
  Cloning into 'build/html'...
  Warning: Permanently added 'github.com,207.97.227.239' (RSA) to the list of known hosts.
  Permission denied (publickey).
  fatal: Could not read from remote repository.
  </code></pre>
  
  <p>But when I do the same tasks locally, everything is OK.</p>
  
  <p>How do I fix this so that the Travis CI build passes and I can still click on the submodule in the repo to direct to it?</p>