Make security-acl an optional dependency

[Tobion]

IMO the symfony/security-acl dependency should be moved from require to require-dev (and suggest).

We are trying to remove ACL from the default symfony installation, see symfony/symfony#14718
And symfony/symfony 3.0 does not require security-acl anymore and security-acl is also extracted into standalone repo since 2.8.

But using symfony-standard 3.0 still installs security-acl because the doctrine cache bundle requires it.

[weaverryan]

👍 - it is only "used" by https://github.com/doctrine/DoctrineCacheBundle/blob/master/Acl/Model/AclCache.php, correct? This is holding up DoctrineBundle's compatibility as well doctrine/DoctrineBundle#486 (comment)

[weaverryan]

Pull request #77