Mismatch type between relay and subscriptions-transport-ws

Hi. I have problem with typing relay subscription.

(I reffered https://github.com/facebook/relay/pull/3062)

Below is the subscribe function with relay.

There is type error.

I resolved the error by modifying `node_modules/subscriptions-transport-ws/dist/client.d.ts`

However it's not reasonable modify I think, since `subscriptions-transport-ws` is not part of relay modules. It is external module.

Also, I tried to like this.
```typescript
return Observable.from(subscribeObservable);
```

I would like to shared my opinion with you.
Thanks.