Restore script to work without pyyaml

[kylef]

As yaml.Loader etc is used in the arguments for the function, even due to our other checks for if yaml the script fails on invocation without first installing pyyaml. This PR fixes the script to work with JSON if you don't have pyyaml installed.

Before

$ ./tools/refract-filter.py test/fixtures/schema/array-fixed-inline-samples.json
Traceback (most recent call last):
  File "./tools/refract-filter.py", line 17, in <module>
    def yaml_load(stream, Loader=yaml.Loader, object_pairs_hook=OrderedDict):
AttributeError: 'NoneType' object has no attribute 'Loader'

After

$ ./tools/refract-filter.py test/fixtures/schema/array-fixed-inline-samples.json
{
  "address": [
    "city"
  ]
}
{
  "$schema": "http://json-schema.org/draft-04/schema#",
  "type": "object",
  "properties": {
    "address": {
      "type": "array",
      "minItems": 3,
      "items": [
        {
          "type": "string"
        },
        {
          "type": "string",
          "enum": [
            "city"
          ]
        },
        {
          "type": "string"
        }
      ],
      "additionalItems": false
    }
  }
}