[RN] Use default param for elastic bounciness

Libraries/Animated/Easing.js

@@ -69,10 +69,7 @@ class Easing {
    *   http://tiny.cc/elastic_b_1 (default bounciness = 1)
    *   http://tiny.cc/elastic_b_3 (bounciness = 3)
    */
-  static elastic(bounciness: number): (t: number) => number {
-    if (arguments.length === 0) {
-      bounciness = 1;
-    }
+  static elastic(bounciness: number = 1): (t: number) => number {
     var p = bounciness * Math.PI;
     return (t) => 1 - Math.pow(Math.cos(t * Math.PI / 2), 3) * Math.cos(t * p);
   };