Symfony compatibility: remove wrongly overridden "load()" method but preserve the logic

[BR0kEN-]

This PR is aimed to fix the next critical issue:

In ServiceContainer.php line 20:

  [Symfony\Component\Debug\Exception\ContextErrorException]
  Warning: Declaration of JonnyW\PhantomJs\DependencyInjection\ServiceContainer::load() should be compatible with Symfony\Component\DependencyInjection\Container::load($file)

By the meaning it duplicates #235 but has no extra changes.