Deprecations in configuration classes

[tuxes3]

Hi, I am using the newest version, currently v1.5, and I am getting two deprecating messages:

A tree builder without a root node is deprecated since Symfony 4.2 and will not be supported anymore in 5.0.
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{▼
  /var/www/vendor/symfony/config/Definition/Builder/TreeBuilder.php:30 {▶}
  /var/www/vendor/pixassociates/sortable-behavior-bundle/Pix/SortableBehaviorBundle/DependencyInjection/Configuration.php:30 {▶}
}

and

The "Symfony\Component\Config\Definition\Builder\TreeBuilder::root()" method called for the "pix_sortable_behavior" configuration is deprecated since Symfony 4.3, pass the root name to the constructor instead.
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/var/www/vendor/symfony/config/Definition/Builder/TreeBuilder.php:51 {▶}
/var/www/vendor/pixassociates/sortable-behavior-bundle/Pix/SortableBehaviorBundle/DependencyInjection/Configuration.php:31 {▶} }

[remigarcia]

Proposed a fix #88