NPM detection doesn't work in ubuntu 14.04

Hello, 

I noticed that on ubuntu:
`not_if "#{node['nodejs']['dir']}/bin/npm -v 2>&1 | grep '#{node['nodejs']['npm']}'"`
resolves to
`not_if "/usr/local/bin/npm -v 2>&1 | grep '1.4.4'"`

However, the `npm` command's full path is: `usr/bin/npm`. This is causing `npm` to install itself every chef client run.

I realize my change might not be ideal because it can pick up another npm install on the same machine. If you have any feedback/suggestions, it would be much appreciated.