Webshot fails to render links or etherpad docs

Saw this in the BugBash today:

```
[2014-03-12T15:25:48.114Z] ERROR: oae/19598 on release0: An uncaught exception was raised to the application.
    Error: spawn ENOENT
        at errnoException (child_process.js:980:11)
        at Process.ChildProcess._handle.onexit (child_process.js:771:34)
[2014-03-12T15:26:18.110Z] ERROR: oae-preview-processor/19598 on release0: Could not generate a screenshot.
    Error: PhantomJS did not respond within the given timeout setting.
        at null._onTimeout (/opt/oae/node_modules/webshot/lib/webshot.js:185:12)
        at Timer.listOnTimeout [as ontimeout] (timers.js:110:15)
[2014-03-12T15:26:18.112Z] ERROR: oae-preview-processor/19598 on release0: Could not generate an image (contentId=c:oae:lJOrjoUgs)
    err: {
      "code": 500,
      "msg": "PhantomJS did not respond within the given timeout setting."
    }
```

There was an update in webshot recently, so fixing the version may be an expedient temporary solution to not block further releases.