Update _visibility.scss

If you use every breakpoint in $breakpoint-classes, the last one will break, because it's impossible to have an "only" range for the highest one. As in, you can't do "xxlarge-only" because there's no upper bound past xxlarge. So the -only classes need to account for this by not printing any CSS.

When printing "small only", the loop should check if the lower bound is 0, and if it is, don't subtract the (1/16)rem, just keep it at 0.