[2.8] Fix lowest deps

Author: nicolas-grekas

composer.json

@@ -23,6 +23,7 @@
     "require-dev": {
         "symfony/phpunit-bridge": "~2.7|~3.0.0",
         "symfony/browser-kit": "~2.4|~3.0.0",
+        "symfony/config": "~2.8|~3.0.0",
         "symfony/console": "~2.7|~3.0.0",
         "symfony/css-selector": "~2.0,>=2.0.5|~3.0.0",
         "symfony/dependency-injection": "~2.6,>=2.6.6|~3.0.0",