Pet Store is serving up teapots

Hey Daniel! Trying out this awesome thing and getting teapot responses instead of valid mocks:

```
docker run -p 8000:8000 danielgtaylor/apisprout https://raw.githubusercontent.com/OAI/OpenAPI-Specification/OpenAPI.next/examples/v3.0/petstore.yaml
```

Responses:

```
http --json GET http://localhost:8000/pets
HTTP/1.1 418 I'm a teapot
Content-Length: 21
Content-Type: text/plain; charset=utf-8
Date: Fri, 27 Jul 2018 14:23:04 GMT

No example available.
```

Any ideas? The `--json` switch sets `Accept: application/json` which I assume is how you determine the responses. Had the same problem in Postman.