Fix for changed ZipArchive in php 5.2.8

We use HHVM and `$zip->open($destination, ZIPARCHIVE::OVERWRITE)` always returned `false` when `undownloaded.zip` didn't exist. This fixes that issue. I think it will work on PHP < 5.2.8 but I'm not totally sure.

http://php.net/manual/en/ziparchive.open.php#88765