Resolve Problem with relocation of Router in Lumen (barryvdh#718)

Due to this commit laravel/lumen-framework@7018d57 on the Lumen 5.5 branch, the core methods required for Routing have been moved to Laravel\Lumen\Routing\Router rather than it residing within the Lumen Application.

src/LumenServiceProvider.php

@@ -14,7 +14,7 @@ class LumenServiceProvider extends ServiceProvider
      */
     protected function getRouter()
     {
-        return $this->app;
+        return $this->app->router;
     }
 
     /**