fix deprecation warnings (>=Symfony 4.3)

[PWalkow]

The "Symfony\Component\Config\Definition\Builder\TreeBuilder::root()" method called for the "welp_mailchimp" configuration is deprecated since Symfony 4.3, pass the root name to the constructor instead.

very similiar to #25

example of fix in doctrine-bundle

$treeBuilder = new TreeBuilder('doctrine');

if (method_exists($treeBuilder, 'getRootNode')) {
    $rootNode = $treeBuilder->getRootNode();
} else {
    // BC layer for symfony/config 4.1 and older
   $rootNode = $treeBuilder->root('doctrine');
}

[Nightbr]

Hey, thanks for the report

Feel free to open a Pull Request for this 👌

[PWalkow]

@Nightbr , sure thing!

[Nightbr]

Fixed in 1.3.2