Make Crypto::random() more reliable on Windows

This patch makes openssl the preferred source of random data on Windows and
will only use mcrypt_create_iv on PHP 5.3.7 and newer.

Fixes #771
Merge pull request #776 from greezybacon/issue/771

Make Crypto::random() more reliable on Windows

Reviewed-By: Peter Rotich <peter@osticket.com>