Add `sudo` to npm updates, edit npm update command

The new way to update npm is `npm install npm -g`, as already indicated here https://github.com/mathiasbynens/dotfiles/pull/423.

In addition, I don't know how everyone else is doing this without getting errors from npm. In my experience I've always had to run npm updates by typing `sudo` beforehand. I think this should be incorporated.

Otherwise, I absolutely love these dotfiles to death... \m/