chore(gesture): null hammer on unlisten

ionic/components/menu/test/multiple/page1.html

@@ -1,6 +1,6 @@
 
 <ion-navbar *navbar>
-  <button [menuToggle]="activeMenu">
+  <button menuToggle>
     <ion-icon name="menu"></ion-icon>
   </button>
   <ion-title>

ionic/gestures/gesture.ts

@@ -65,11 +65,12 @@ export class Gesture {
       this._hammer.destroy();
     }
     this._callbacks = {};
+    this._hammer = null;
     this.isListening = false;
   }
 
   destroy() {
     this.unlisten();
-    this._hammer = this.element = this._options = null;
+    this.element = this._options = null;
   }
 }