Update pyramid.scad

There is a display bug when using difference() together with polyhedron() and when the model is "insideout". See https://github.com/openscad/openscad/issues/228

Changed the triangles direction into counter-clockwise. 

Test:
module pyramid(w, l, h) {
	mw = w/2;
	ml = l/2;
	polyhedron(points = [
		[0, 0, 0],
		[w, 0, 0],
		[0, l, 0],
		[w, l, 0],
		[mw, ml, h]
	], triangles = [
		[4, 1, 0],
		[4, 3, 1],
		[4, 2, 3],
		[4, 0, 2],
	//base
		[0, 1, 2],
		[2, 1, 3]
	]);
}

difference() {
   pyramid(61,18,200);
   translate([0,0,3]) cube([61,18,200]);
 }