fix for users of Symfony 3 dir strcture without breaking backward compat...

Symfony 2.5 allows the use of the new Symfony 3 dir structure. 

In this new structure, amongst other things, the bootstrap.cache.php file has been moved to the /var directory, with the AppKernel.php remaining in the /app dir. 

The Symfony2 module currently doesn't cater for having these two files in different dirs. 

However, there is a confusing error message in Codeception\Module\Symfony2::getKernelClass which hints at two different config params:

app_path

and

app_dir

Adding in both params to the *.suite.yml file resolves this (app_path: var, app_dir: app) but breaks backwards compatibility by requiring the new param.

This commit should enable users of the new dir structure to use an additional param to specify the location of the var dir, whilst not breaking anything for existing 2.x users.