Update Encrypt.php

* Not too long ago, @defuse informed me that `MCRYPT_DEV_URANDOM` on Windows works as well as it does on BSD/Linux.
* Don't allow insecure, predictable random numbers for IVs
Update Encrypt.php
Update Encrypt.php
Merge pull request #540 from sarciszewski/patch-3

Update Encrypt.php: do not allow the insecure random number generator MCRYPT_RAND