Use `n` to manage node versions on RPi

[jweisz]

Seriously. Why did they call it n?
https://github.com/tj/n

This seems preferable to use over nvm because when we run scripts with sudo (which we need to when using the LED), the environment variables set by nvm to point to the current node install don't get carried over. Even with sudo -E. So then the scripts run on an older version of node and they complain that all the modules were compiled for a newer version of node. It's a mess.

Install n with:

curl -L https://raw.githubusercontent.com/tj/n/master/bin/n -o n

[jweisz]

Also we should add installing n to bootstrap, plus add documentation somewhere that this is how we're managing node on RPi.