Don't try to dereference non-ref properties called $ref

[kylef]

Fixes parsing Swagger documents that contain properties called $ref. Previously the parser would attempt to dereference the property as would generically try to dereference any key in an object called $ref, as per the JSON Schema specification, references are only permitted when a schema type is expected.

For example, the following is a schema which is trying to describe an object with a property called $ref, previously the parser would attempt to dereference $ref and crash in the process.

  type: object
  properties:
    $ref:
      type: string
      example: '#/definitions/User'
  required: ['$ref']

There is a still an open known issue #235 that uses of $ref in a Swagger 2 document where the value a string will cause the parser to attempt to dereference it incorrectly when the $ref property is found on an object that does not support $ref as per the Swagger 2 and JSON Schema specifications. The proposed fix in this PR is a "quick fix" for the crash, fixing this for all cases is going to be a substancial amount of refactoring as the way Swagger-parser and our code is dereferencing doesn't limit $ref only into places in the Swagger object where $ref is supported. My fix will work for the majority of cases, for example the following Swagger which previously caused a crash:

swagger: '2.0'
info:
  version: '1.0'
  title: My API
produces:
- application/json
paths:
  /:
    get:
      responses:
        200:
          description: Returns a JSON Pointer
          schema:
            type: object
            properties:
              $ref:
                type: object
                example: '#/definitions/User'