Security docs recommend usage of deprecated class

[zsturgess]

http://symfony.com/doc/current/components/security/secure_tools.html

In the 3.0+ version of the docs, use of the deprecated Symfony\Component\Security\Core\Util\SecureRandom class is still recommended.

This needs to be updated to talk about the random_bytes() function and the paragonie/random_compat package, the recommended replacement.