Switch to NodeSource repository

- Chris Lea's PPA will be dropped with the release of 0.12.x
- https://chrislea.com/2014/07/09/joining-forces-nodesource/
- https://nodesource.com/blog/chris-lea-joins-forces-with-nodesource
Merge pull request #26 from engineyard-cookbooks/nodesource

Switch to NodeSource repository