Added support of any BrowserKit client with BrowserKit 2.3+

BrowserKit 2.3 introduced ``Client::getInternalResponse`` to access the BrowserKit response in all cases, as changing ``getResponse`` was impossible for BC reasons. Using this method will ensure that that driver can work with any client implementation assuming 2.3+ is used.
I also added a safeguard against accessing the response before visiting a page, to avoid getting a fatal error in such case.

The 2.x version of driver should then drop the support of Symfony 2.2 and lower (which are not maintained anymore) to simplify the code.