Replace Chris Lea's PPA with the Nodesource repo

According to Chris's [blog post](https://chrislea.com/2014/07/09/joining-forces-nodesource/) or the [Nodesource one](https://nodesource.com/blog/chris-lea-joins-forces-with-nodesource), from July he joined the NodeSource team and he:
> (...) will continue to support his PPA on Launchpad for 0.10 but will not be supporting Node.js v0.12 and beyond.
> The new repository will be located at https://deb.nodesource.com (...)

In other words, Chris's PPA is dead, long live the Nodesource repo.