fix register_shutdown_function() call for HHVM 4.27+

Author: jjergus

src/_Private/Filesystem.hack

@@ -16,7 +16,7 @@ final class Filesystem {
   ): string {
     $fname = \tempnam(\sys_get_temp_dir(), $prefix);
     if ($cleanup) {
-      \register_shutdown_function(array(Filesystem::class, 'remove'), $fname);
+      \register_shutdown_function(() ==> Filesystem::remove($fname));
     }
     return $fname;
   }