Update hal-schema.json for JSON Schema Draft v04

The current version did not validate using [TinyValidator](https://github.com/geraintluff/tv4) or [JaySchema](https://github.com/natesilva/jayschema) which use the current JSON Schema Draft v04. 

Example:

````
{
  "message": "Welcome",
  "_links": {
    "self": {
      "href": "http://localhost/"
    }
  }
}
````

…which is valid HAL but would produce the following error when checked with a JSON Schema v04 validator against this schema:

````
[ { [Error]
    instanceContext: '#/_links/self',
    resolutionScope: 'resource#/properties/_links/additionalProperties',
    constraintName: 'type',
    constraintValue: [ [Object], [Object] ],
    testedValue: 'object' } ]
````
(example from JaySchema, same error with tv4)

The commit changes the schema and removes the array in the additionalProperties definition for the _links object which makes it validate using a JSON Schema v04 validator.