Made the bundle compatible with Symfony 2.3 LTS again

Forbidding to use SensioFrameworkExtraBundle 3.0 on 2.3 and 2.4 is a bad idea, given that people want to use the new features.

Here is the current state:

- using ``@Method`` on a class will only work in Symfony 2.5+. It will be ignored for older versions of Symfony (just like when using 3.0.0 of this bundle)
- the ``security:check`` command will only work in Symfony 2.4+ as it is registered as service. It will not be registered for 2.3 users (we could find a way to register it in 2.3, but the blog announces it as a new 2.6 feature anyway)
- using ``@Security`` requires Symfony 2.4+
- using the ``lastModified`` or ``etag`` properties of ``@Cache`` requires installing the ExpressionLanguage component (you can install it in a 2.3 project as it is totally standalone)