Update dependency to use symfony/service-contracts to prevent autoloading clashes with Symfony 4.3

Author: wbrinkman

composer.json

@@ -25,7 +25,7 @@
         "symfony/config": "^3.4 || ^4.0",
         "symfony/dependency-injection": "^3.4 || ^4.0",
         "symfony/http-kernel": "^3.4 || ^4.0",
-        "symfony/contracts": "^1.0"
+        "symfony/service-contracts": "^1.0"
     },
     "require-dev": {
         "friendsofphp/php-cs-fixer": "^2.14",