replace udev BUS with SUBSYSTEM

BUS was removed somewhere after udev version 149. It can be replaced by SUBSYSTEM.
Source: http://sdrblog.wordpress.com/2010/02/01/new-udev-rules/
udev looks for parent's ATTRS to match child

When matching against ATTR, udev will match the raw usb node directly, but we want to modify the device that is using the usbtmc driver (the toplevel child /dev/usbtmc0). This will be matched, when matching against parents ATTRS and SUBSYSTEMS.
Merge pull request #1 from niun/master

update README for newer udev versions