Update to match latest version of Hammer

Internals of Hammerjs [changed](https://github.com/EightMedia/hammer.js/blob/fc6b1c23b61be607650497f4902ad8c4abd5e7e2/src/instance.js#L39)

Author: rdsubhas

index.js

@@ -18,9 +18,10 @@ exports.install = function (Vue) {
         },
         unbind: function () {
             this.el.hammer.off(this.arg, this.handler)
-            if (!this.el.hammer._eventHandler.length) {
+            if (!this.el.hammer.eventHandlers.length) {
+                this.el.hammer.dispose()
                 this.el.hammer = null
             }
         }
     })
-}
+}