PHP 7 incompatibility

I've updated to php7.0 and last version 1.25.6. And still have this issue :

```
PHP Fatal error:
'message' => 'Cannot use \'String\' as class name as it is reserved',
'file' => '/var/app/current/vendor/hypercharge/json-schema-php/src/JsonSchema/Constraints/String.php',
'line' => 18
```