Declaration of JonnyW\..\ServiceContainer::load() should be compatible with Symfony\..\Container::load($file)

Issue:

`Declaration of JonnyW\PhantomJs\DependencyInjection\ServiceContainer::load() should be compatible with Symfony\Component\DependencyInjection\Container::load($file)`

Ducktape: 

`JonnyW\PhantomJs\DependencyInjection\ServiceContainer::load(`**$file = null**`)`