use pageX/YOffset instead of scrollX/Y

As pointed out by @pliasetski these properties are not supported in IE11 and below

This fixes #409

src/js/plugin/handler/click-rail.js

@@ -19,7 +19,7 @@ function bindClickRailHandler(element, i) {
   }
   i.event.bind(i.scrollbarYRail, 'click', function (e) {
     var halfOfScrollbarLength = h.toInt(i.scrollbarYHeight / 2);
-    var positionTop = i.railYRatio * (e.pageY - window.scrollY - pageOffset(i.scrollbarYRail).top - halfOfScrollbarLength);
+    var positionTop = i.railYRatio * (e.pageY - window.pageYOffset - pageOffset(i.scrollbarYRail).top - halfOfScrollbarLength);
     var maxPositionTop = i.railYRatio * (i.railYHeight - i.scrollbarYHeight);
     var positionRatio = positionTop / maxPositionTop;
 
@@ -40,7 +40,7 @@ function bindClickRailHandler(element, i) {
   }
   i.event.bind(i.scrollbarXRail, 'click', function (e) {
     var halfOfScrollbarLength = h.toInt(i.scrollbarXWidth / 2);
-    var positionLeft = i.railXRatio * (e.pageX - window.scrollX - pageOffset(i.scrollbarXRail).left - halfOfScrollbarLength);
+    var positionLeft = i.railXRatio * (e.pageX - window.pageXOffset - pageOffset(i.scrollbarXRail).left - halfOfScrollbarLength);
     var maxPositionLeft = i.railXRatio * (i.railXWidth - i.scrollbarXWidth);
     var positionRatio = positionLeft / maxPositionLeft;