Fixed issue with Configuration that was not enforcing default_sender to be present.

Author: Guilherme Blanco

DependencyInjection/Configuration.php

@@ -31,6 +31,7 @@ public function getConfigTreeBuilder()
                 ->arrayNode('composer')
                     ->children()
                         ->arrayNode('default_sender')
+                        ->isRequired()
                             ->children()
                                 ->scalarNode('name')
                                     ->isRequired()