Fixes #452: Use is_file instead of file_exists when checking for sheb…

…ang files.

src/Runner.php

@@ -244,7 +244,7 @@ protected function shebang($args)
      */
     protected function isShebangFile($filepath)
     {
-        if (!file_exists($filepath)) {
+        if (!is_file($filepath)) {
             return false;
         }
         $fp = fopen($filepath, "r");