Revert "Bump avr core version to 1.6.13"

This must be made together with the release of AVR core 1.6.13.

This reverts commit d20e608.

Author: cmaglie

hardware/arduino/avr/platform.txt

@@ -6,7 +6,7 @@
 # https://github.com/arduino/Arduino/wiki/Arduino-IDE-1.5-3rd-party-Hardware-specification
 
 name=Arduino AVR Boards
-version=1.6.13
+version=1.6.12
 
 # AVR compile variables
 # ---------------------