Disable test that dumps hex codes for generated BSON

There is an equivalent test in Hippo:
tests/bson-serialize-002.phpt which tests the same, with the __pclass encoded
last.

Author: derickr

tests/bson/bson-encode-004.phpt

@@ -1,6 +1,7 @@
 --TEST--
 BSON encoding: Object Document Mapper
 --SKIPIF--
+<?php if (defined("HHVM_VERSION_ID")) exit("skip HHVM encodes __pclass last"); ?>
 <?php require __DIR__ . "/../utils/basic-skipif.inc"?>
 --FILE--
 <?php