Fix commands

[7ochem]

Most important changes:

• In src/Command/DumpCommand.php:

  -class DumpCommand extends ContainerAwareCommand
  +class DumpCommand extends Command

• Added "symfony/console": "^4.4", in composer.json
• Pass root name to TreeBuilder constructor and call getRootNode instead of deprecated ->root() in src/DependencyInjection/Configuration.php