fixed issue with delete operator on stacks (#218)

* fixed issue with delete operator 

using delete operator in arrays ,element remains undefined. so splice is the clean way to do that

* Fixed the semicolon and let errors

* Update Stack.js

Data-Structures/Stack/Stack.js

@@ -30,7 +30,7 @@ var Stack = (function () {
 
     this.top--
     var result = this.stack[this.top]
-    delete this.stack[this.top]
+    this.stack = this.stack.splice(0, this.top)
     return result
   }