Fix array children in PHP5 and HHVM without EnableHipHopSyntax

Array isn't usually Traversable.

Fixes facebook/hhvm#2631

```
[fredemmott@devbig076 /tmp/xhp-bs] hhvm -v Eval.EnableXHP=1 -v
Eval.EnableHipHopSyntax=0 test.php
string(10) "<b>foo</b>"
string(10) "<b>foo</b>"
[fredemmott@devbig076 /tmp/xhp-bs] hhvm -v Eval.EnableXHP=1 -v
Eval.EnableHipHopSyntax=1 test.php
string(10) "<b>foo</b>"
string(10) "<b>foo</b>"
[fredemmott@devbig076 /tmp/xhp-bs] cat test.php
<?php

require('/home/fredemmott/fio/xhp/php-lib/init.php');
var_dump((string) <b>{['foo']}</b>);
var_dump((string) <b>{new ArrayIterator(['foo'])}</b>);
```