`--max-idle=n` should now be added for Puma

[davidalejandroaguilar]

The readme mentions:

--max-idle=n - exit afer n seconds of inactivity. Supports iso 8601 and sleep syntaxes. Uses passenger for now, awaiting puma/puma#2580 support.

Puma has just added Shutdown on Idle on puma/puma#3209 🎉