use window height for viewport height instead of document height

src/skrollr.js

@@ -506,7 +506,7 @@
 	 * That is, calculate anchor position and offset of element.
 	 */
 	Skrollr.prototype.relativeToAbsolute = function(element, viewportAnchor, elementAnchor) {
-		var viewportHeight = documentElement.clientHeight;
+		var viewportHeight = window.innerHeight;
 		var box = element.getBoundingClientRect();
 		var absolute = box.top;
 
@@ -1636,4 +1636,4 @@
 
 	//Animation frame id returned by RequestAnimationFrame (or timeout when RAF is not supported).
 	var _animFrame;
-}(window, document));
+}(window, document));