lots of issues trying to generate an api

Hi, I'm super excited to use this pkg, but I'm having lots of issues. I'd also like to help where I can.

I'm using this open api v3 spec https://us-east-1-production-zcv2-api.ziff.ai/_schema

first problem is:
```
ERROR $root.components.schemas.cloud.schema.FileEncoder.schema.properties.encoderModelUri.schema has invalid property: $ref
ERROR $root.components.schemas.cloud.schema.FileEncoder.schema.properties.encoderModelUri.reference has invalid property: readOnly
ERROR $root.components.schemas.cloud.schema.FileEncoder.reference is missing required property: $ref
ERROR $root.components.schemas.cloud.schema.FileEncoder.reference has invalid properties: type, required, properties
```

As I wasn't using that model, I just removed that one field in the json spec.

My next issue was

```
2019/02/07 09:22:57 unimplemented: any_of: > any_of: > any_of: >
2019/02/07 09:22:57 unimplemented: any_of: > any_of: >
```

I understand that anyOf types would be hard to implement in go, I just changed these to type string

`gnostic ziff-api.json --go-generator-out=ziff` then ran without errors but types.go and client.go were completely empty.