Fixed stacking not working (due to a small bug in the PR for rtl support)

lib/timeline/Stack.js

@@ -111,20 +111,19 @@ exports.nostack = function(items, margin, subgroups) {
  * @param {{horizontal: number, vertical: number}} margin
  *                          An object containing a horizontal and vertical
  *                          minimum required margin.
+ * @param {boolean} rtl
  * @return {boolean}        true if a and b collide, else false
  */
 exports.collision = function(a, b, margin, rtl) {
-  var isCollision = null;
   if (rtl) {
-    isCollision = ((a.right - margin.horizontal + EPSILON)       < (b.right + b.width) &&
+    return  ((a.right - margin.horizontal + EPSILON)  < (b.right + b.width) &&
     (a.right + a.width + margin.horizontal - EPSILON) > b.right &&
     (a.top - margin.vertical + EPSILON)              < (b.top + b.height) &&
     (a.top + a.height + margin.vertical - EPSILON)   > b.top);
   } else {
-    ((a.left - margin.horizontal + EPSILON)       < (b.left + b.width) &&
+    return ((a.left - margin.horizontal + EPSILON)   < (b.left + b.width) &&
     (a.left + a.width + margin.horizontal - EPSILON) > b.left &&
     (a.top - margin.vertical + EPSILON)              < (b.top + b.height) &&
     (a.top + a.height + margin.vertical - EPSILON)   > b.top);
   }
-  return isCollision;
 };