Update composer.json

To prevent the following problem after a composer update:

Fatal error: Uncaught exception 'Symfony\Component\Config\Definition\Exception\InvalidConfigurationException' with message 'Unrecognized options "mappings" under "cmf_routing_auto"' in /Users/jeremy/Projects/cmf/vendor/symfony/symfony/src/Symfony/Component/Config/Definition/ArrayNode.php:316 Stack trace: #0 /Users/jeremy/Projects/cmf/vendor/symfony/symfony/src/Symfony/Component/Config/Definition/BaseNode.php(268): Symfony\Component\Config\Definition\ArrayNode->normalizeValue(Array) #1 /Users/jeremy/Projects/cmf/vendor/symfony/symfony/src/Symfony/Component/Config/Definition/Processor.php(33): Symfony\Component\Config\Definition\BaseNode->normalize(Array) #2 /Users/jeremy/Projects/cmf/vendor/symfony/symfony/src/Symfony/Component/Config/Definition/Processor.php(50): Symfony\Component\Config\Definition\Processor->process(Object(Symfony\Component\Config\Definition\ArrayNode), Array) #3 /Users/jeremy/Projects/cmf/vendor/symfony-cmf/routing-auto-bundle/Symfony/Cmf/Bundle/RoutingAutoBundle/DependencyInjection/CmfRoutingAutoExtens in /Users/jeremy/Projects/cmf/vendor/symfony/symfony/src/Symfony/Component/Config/Definition/ArrayNode.php on line 316