Added support for serverVersion connection parameter

Doctrine DBAL 2.5 will support platform version autodetection, additionally it allows specifying serverVersion connection parameter:

> which the user can pass to skip platform autodetection and define the desired platform version straight away. This is also required for drivers that cannot return the database server version without an additional query (performance reasons).
> https://github.com/doctrine/dbal/pull/487

In symfony it can be useful eg. when warming up cache during build, so you don't need the db connection, but dbal 2.5 will try to autodetect server version anyway eg. during entity class metadata loading, then it will throw an exception because of unavailable connection. ServerVersion parameter can be used to skip the autodetection.
please use an underscored version in the config, to fit the Symfony convention in YAML and XML files.

the name should then be mapped to the camelCased key used by Doctrine in the DI extension (we already do it a lot)