No longer de-escalate the PhantomJS user.

The de-escalation is removed. Updated the README to reflect a better installation. It's also preferred because:

1. `node.js` is no longer running as root.
2. `phantomjs` doesn't need a odd-man `sandbox` user. Images will be generated and owned by the user the app should be running under, a non-privileged user.
3. Additionally, culled out the `/opt/bin/phantomjs` path so users may use different install paths.