Found a moment to install Arduino 1.6.6, it would appear that a manual include of "Keyboard.h" is now required, rather than the previous automatic inclusion. I've no idea when this happened, but it's an easy fix.
The IDE even suggests it in the error output:
Picade:88: error: 'Keyboard' not found. Does your sketch include the line '#include <Keyboard.h>'?
So, just add:
#include <Keyboard.h>
And now it compiles for me!