publish avr core 1.6.16 prerelease

remember to correct the core size and checksum when pushing the official core

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=1.6.15
+version=1.6.16
 
 # AVR compile variables
 # ---------------------