Error installing on SF3.4

Afer installing this package I get the following error?

Fatal error: Uncaught Symfony\Component\Debug\Exception\UndefinedMethodException: Attempted to call an undefined method named "getRootNode" of class "Symfony\Component\Confi
g\Definition\Builder\TreeBuilder". in /Users/ceesvanegmond/Sites/asdfasf/vendor/api-postcode/api-postcode-bundle/DependencyInjection/Configuration.php:27

Any idea?