Fixed issue with SassColour RGB to HSL

As pointed out by @DCollinsSA

Line 861 "$s = $l > 0.5 ? $d / (2 - $max - $min) : $d / ($max + $min);" must be put in a conditional that checks if ($max+$min) > 0, else an Exception will be thrown when trying to convert the color black to HSL.

Signed-off-by: Rudy Affandi <rudy@adnetinc.com>