text: Title: Missing 'url' key raises uncaught KeyError on coveralls.io 503 response
  Body: Could not reproduce with `coveralls debug` as coveralls.io had evidently fixed its server-side error by this time.  Traceback from original run below:
  
  ```
  Submitting coverage to coveralls.io...
  Coverage submitted!
  Failure to submit data. Response [503]: <!DOCTYPE html>
      <html>
      <head>
        <style type="text/css">
          html, body, iframe { margin: 0; padding: 0; height: 100%; }
          iframe { display: block; width: 100%; border: none; }
        </style>
      <title>Application Error</title>
      </head>
      <body>
        <iframe src="https://s3.amazonaws.com/assets.coveralls.io/maintenance.html">
          <p>Application Error</p>
        </iframe>
      </body>
      </html>
  Traceback (most recent call last):
    File "/home/rof/.virtualenv/bin/coveralls", line 9, in <module>
      load_entry_point('coveralls==0.4.1', 'console_scripts', 'coveralls')()
    File "/home/rof/.virtualenv/local/lib/python2.7/site-packages/coveralls/cli.py", line 52, in main
      log.info(result['url'])
  KeyError: 'url'
  ```
  