Add legacy version of symfony/config for PHP 7.0

Author: bbatsche

composer.json

@@ -8,7 +8,7 @@
         "php": "^7.0",
         "ext-json": "*",
         "composer/composer": "@stable",
-        "symfony/config": "^4.4",
+        "symfony/config": "^2.8||^4.4",
         "symfony/console": "^2.8||^4.0",
         "symfony/dependency-injection": "^2.8||^4.3",
         "symfony/process": "^2.1||^4.1"