Add Symfony Contracts Dependency

Author: fionera

composer.json

@@ -24,7 +24,8 @@
         "symfony/asset": "^3.4 || ^4.0",
         "symfony/config": "^3.4 || ^4.0",
         "symfony/dependency-injection": "^3.4 || ^4.0",
-        "symfony/http-kernel": "^3.4 || ^4.0"
+        "symfony/http-kernel": "^3.4 || ^4.0",
+        "symfony/contracts": "^1.0"
     },
     "require-dev": {
         "friendsofphp/php-cs-fixer": "^2.13",