Support the required format of the latest schema version

The latest JSON Scheme has changed, it says the required flag can only be included in object type schemas, and the following applies:

```
5.4.3.  required

5.4.3.1.  Valid values

The value of this keyword MUST be an array. This array MUST have at least one element. Elements of this array MUST be strings, and MUST be unique.

5.4.3.2.  Conditions for successful validation

An object instance is valid against this keyword if its property set contains all elements in this keyword's array value.
```

This patch makes it possible to use both styles, but doesn't do much validation for input. Let me know if you're interested in merging and if I need to clean it up. This is a bit of a showstopper for me, because I don't want to use the outdated schema style for new project and still want to get the required fields processed correctly.