function gammaDeltaRatio( z, delta ) {
	return addon( z, delta );
}
Computes the ratio of two gamma functions.

@private
@param {number} z - first gamma parameter
@param {number} delta - difference
@returns {number} gamma ratio

@example
var y = gammaDeltaRatio( 2.0, 3.0 );
// returns ~0.042

@example
var y = gammaDeltaRatio( 4.0, 0.5 );
// returns ~0.516

@example
var y = gammaDeltaRatio( 100.0, 0.0 );
// returns 1.0