Fix yaml as @ is forbidden in YAML 1.2

http://www.yaml.org/spec/1.2/spec.html#id2772075

(PR done on `2.5` branch, but this issue persists also in `2.6`, `2.7` and `master`)

Author: jdespatis

app/config/routing.yml

@@ -1,3 +1,3 @@
 app:
-    resource: @AppBundle/Controller/
+    resource: "@AppBundle/Controller/"
     type:     annotation