Support Teensy 3.0 & 3.1 using AVR code

This library's AVR code works great on Teensy 3.0 & 3.1, because Teensy emulates the necessary AVR registers.  Unfortunately, the recent changes for Arduino Due cause the AVR code to no longer be used with compiling for Teensy3.

Please consider merging this tiny patch.  It merely enables the AVR code to be used again for Teensy3.