Update OkHttp to 3.4.0 and Okio to 1.9.0

Summary:
- 3.3.1 wasn't compatible with Robolectric 3.0, however 3.4.0 [fixes](https://github.com/square/okhttp/issues/2533#issuecomment-230198063) this issue.
- Few other fixes to OkHttp mentioned in the [CHANGELOG.md](https://github.com/square/okhttp/blob/master/CHANGELOG.md)

> Fixes: #7743
>
- Android apps can recover from a `REFUSED_STREAM` in HTTP/2.
- A few other fixes mentioned in the [CHANGELOG.md](https://github.com/square/okhttp/blob/master/CHANGELOG.md)

- CircleCi
- Test with `/Examples` ✅
Closes https://github.com/facebook/react-native/pull/8672

Reviewed By: alsutton

Differential Revision: D3541293

Pulled By: bestander

fbshipit-source-id: 76429861b4f4df15cb9c18ab0f177daee3e1459d