Update bundled avr core to 1.6.15

Author: facchinm

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=1.6.14
+version=1.6.15
 
 # AVR compile variables
 # ---------------------