Fix a copy paste error (probably)

I don't use this helper, however, I guess that line wasnt correct.

src/Generator.php

@@ -71,7 +71,7 @@ class Generator
     protected function detectDrivers()
     {
         try{
-            if (class_exists('Cache')) {
+            if (class_exists('Auth')) {
                 $class = get_class(\Auth::driver());
                 $this->extra['Auth'] = array($class);
                 $this->interfaces['\Illuminate\Auth\UserProviderInterface'] = $class;