Release avr 1.4.15

[hathach]

• release 1.4.15 to add Wire::end() support
• this pr should be re-based and merged after #90 to prevent conflict

[hathach]

@ladyada please review this one, this is release for changes to avr board in https://github.com/adafruit/Adafruit_Arduino_Boards/pull/19