mcrypt_create_iv() is not available in PHP>7.2

Hello,

in PHP>7.2 there is no possibilities to use `mcrypt_create_iv()` function (athens/encryption/src/Cipher.php:41). The solution could be to replace 
```php
$iv = mcrypt_create_iv(self::IV_SIZE, MCRYPT_RAND);
```
with
```php
$iv = openssl_random_pseudo_bytes(self::IV_SIZE);
```