Merge pull request #216 from JMLamodiere/cannot-be-empty-deprecated

cannotBeEmpty on arrayNode is deprecated in symfony 3 and removed in 4

Author: kbond

DependencyInjection/Configuration.php

@@ -59,7 +59,7 @@ public function getConfigTreeBuilder()
                     ->canBeEnabled()
                     ->children()
                         ->arrayNode('recipient')
-                            ->isRequired()->cannotBeEmpty()
+                            ->isRequired()->requiresAtLeastOneElement()
                             ->prototype('scalar')->end()
                             ->beforeNormalization()
                                 ->ifString()