Attempted to call function "mcrypt_encrypt" from namespace "Crevillo\Payum\Redsys".

This error appears when I want to use `redsys` for testing.
PHP 7.2 doesn't support this function already. So you have to change logic a bit. Thanks.

And advice: better take some specific versions of images, which stable for 100% rather than having latest php, nginx etc. releases.
