Fix to install failure's on Ubuntu

Hi.

As described here: https://chrislea.com/2013/03/15/upgrading-from-node-js-0-8-x-to-0-10-0-from-my-ppa/, the node.js package in Chris Lea's PPA now contains npm. Requiring npm separately causes Ubuntu to fallback to it's own packaged version, which requires an earlier version of node.js. This patch causes the npm package to not be required if using Chris's PPA.