Fix FEEDING_INTERVAL truncation (16bit).

Many compilers will auto-promote or warn, rather than silently truncate
arithmetic literals. Not so the AVR compiler. Designating all numbers as longs
fixed the truncation issue.
Adding time correction based on six-minute skew.

Basic measurements of six minute intervals (relative to Trinket msecs), yielded
5m52s of wall-clock time. This is scaled down to msec proportions for smoother
clock adjustment.

The adjustment mechanism uses long math carefully to avoid under/overflow
cases, particularly near 32-bit msec wrap-around.