Use patternProperties for $ref

While draft-wright-json-schema-00 forbids JSON References except
where a schema is expected, no validators implement this until
draft-writght-json-schema-01.  wright-00 schemas are in practice
processed as draft-04 schemas, meaning that "$ref" is always
considered a JSON Reference.  Or, at least, some validators do
that.

We can work around this by using a pattern that only matches "$ref".
There is no problem with the $ref in the "required" keyword, as
only "$ref" as a property name is recognized as a reference.