Make dependency on contracts explicit

Will not fix #122 but should prevent similar issues.

Author: curry684

composer.json

@@ -21,6 +21,7 @@
     },
     "require": {
         "php": "^7.2",
+        "symfony/contracts": "^1.1|^2.0",
         "symfony/framework-bundle": "^4.1|^5.0",
         "symfony/property-access": "^4.1|^5.0",
         "symfony/translation": "^4.1|^5.0"