[YAML] spec example 5.3 is not parsed

http://www.yaml.org/spec/1.2/spec.html#id2772312

``` yaml
sequence:
- one
- two
mapping:
  ? sky
  : blue
  sea : green
```

Code above (taken from spec) results in `Symfony\Component\Yaml\Exception\ParseException` with message `Unable to parse at line 6 (near "? sky").`
