Update config format_listener rules to be array prototype fixes #589

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #589
| License       | MIT
| Doc PR        | update

Yaml dumper support in symfony/symfony misses this point i think while dumping prototype arrays, this manually fixes the proper way of configuring the rules under format_listener.

great talk guys @willdurand & @lsmith77