Add ko.untrack() to knockout.es5

See https://github.com/SteveSanderson/knockout-es5/blob/75773a52f543380aa6dbf6ccffb4b8fc56fa8a2a/src/knockout-es5.js#L196

knockout.es5/knockout.es5-tests.ts

@@ -54,6 +54,11 @@ class OrderLine {
     public getSubtotal(): string {
         return "$" + (this.price * this.quantity).toFixed(2);
     }
+
+    public dispose() {
+        // Dispose of all the observables of this object to prevent memory leaks
+        ko.untrack(this);
+    }
 }
 
 class Order {
@@ -83,4 +88,4 @@ anOrder.lines.removeAll();
 
 anOrder.lines.destroy(someOrderLine);
 anOrder.lines.destroyAll([someOrderLine]);
-anOrder.lines.destroyAll();
+anOrder.lines.destroyAll();

knockout.es5/knockout.es5.d.ts

@@ -7,6 +7,7 @@
 
 interface KnockoutStatic {
     track(obj: any, propertyNames?: Array<string>): any;
+    untrack(obj: any, propertyNames?: Array<string>): any;
     defineProperty(obj: any, propertyName: string, evaluator: Function): any;
     defineProperty(obj: any, propertyName: string, options: KnockoutDefinePropertyOptions): any;
     getObservable(obj: any, propertyName: string): KnockoutObservable<any>;
@@ -26,4 +27,4 @@ interface Array<T> {
 	destroy(item: T): void;
 	destroyAll(items: T[]): void;
 	destroyAll(): void;
-}
+}