The error is caused by using Arduino AVR Boards 1.6.10 with Arduino IDE 1.6.6, they are incompatible. The solution is to either:
- Upgrade to Arduino IDE 1.6.7 or 1.6.8
- Install Arduino AVR Boards 1.6.9 using Boards Manager(Tools > Board > Boards Manager... > Arduino AVR Boards(click on it) > Select version > 1.6.9 > Install) - I've had problems reinstalling the previous Arduino AVR Boards version with Boards Manager so you may have to reinstall the Arduino IDE if you can't compile after going back to Arduino AVR Boards 1.6.9.
If you're using Windows then you can fix the problem by installing Arduino IDE 1.6.5r5 as DrAzzy said but with other OSs this might cause another problem because Arduino AVR Boards 1.6.10 is also incompatible with Arduino IDE 1.6.5r5 and that boards package will still be installed.