fix(router): remove app from $router.apps array once app destroyed (Fixes vuejs#2639)

Prevent destroyed apps from causing memory leak in `$router.apps` array.

Fixes vuejs#2639

Author: tmorehouse

Diff for: src/index.js

@@ -91,7 +91,12 @@ export default class VueRouter {
 
     // main app already initialized.
     if (this.app) {
-      return
+     app.$once('hook:destroyed', () {
+       // Clean out app from this.apps array once destroyed
+       const index = this.apps.indexOf(app)
+       if (index > -1) this.apps.splice(index, 1)
+     })
+     return
     }
 
     this.app = app