Delete existing links that are broken

When a link exists but is broken, `file_exists($link)` return false. And
when `symlink($link, $target)` is called on a broken link, a PHP Warning
is returned and the link is not updated. To fix this, we add an
additional check using `is_link($link)` (which return true, even if the
link is broken) to detect and delete broken links.

src/Illuminate/Foundation/Console/StorageLinkCommand.php

@@ -31,6 +31,10 @@ public function handle()
             return $this->error('The "public/storage" directory already exists.');
         }
 
+        if (is_link(public_path('storage'))) {
+            $this->laravel->make('files')->delete(public_path('storage'));
+        }
+
         $this->laravel->make('files')->link(
             storage_path('app/public'), public_path('storage')
         );