Change FRACT_INC from X/250 to X/256

[nerdralph]

FRACT_INC is an 8-bit binary fraction with a denominator of 250.
https://github.com/nerdralph/ArduinoShrink/blob/master/src/millis.c#L35

Changing to 256 would slightly improve micros() precision, and simplify T0 overflow ISR, removing the need for FRACT_MAX.