.get() hangs if /usr/sbin/ isn't on $PATH

[amaramth]

Fix: change var cmd = 'ioreg'; (https://github.com/gillstrom/osx-brightness/blob/d40f29b3356b8ce62115c88b2d2f6e105f1ff754/index.js#L28) to var cmd = '/usr/sbin/ioreg';