Fix method signature in Doctrine\DBAL\Driver\Connection

The method ``prepare`` must have an optional driverOptions parameter to be compatible with class which implement the interface Doctrine\DBAL\Driver\Connection.

To avoid this problem:

HipHop Fatal error: Declaration of Doctrine\DBAL\Driver\PDOConnection::prepare() must be compatible with that of Doctrine\DBAL\Driver\Connection::prepare() in /home/travis/build/symfony/symfony/vendor/doctrine/dbal/lib/Doctrine/DBAL/Driver/PDOConnection.php on line 30