Deprecated Unescaper is being used Yaml itself

It's being used here https://github.com/symfony/symfony/blob/2.7/src/Symfony/Component/Yaml/Inline.php#L263

And causing

```
[ErrorException]
Constant ENCODING in class Symfony\Component\Yaml\Unescaper is deprecated since version 2.5 and will be removed in 3.0.
```
