Update retrofit to 1.8.0.

- https://github.com/square/retrofit/blob/master/CHANGELOG.md#version-180-2014-11-18
Update okhttp to 2.1.0.

- https://github.com/square/okhttp/blob/master/CHANGELOG.md#version-210
- okhttp is now required.
- Update readme dep version numbers.
Prepare version 3.4.0.
Changes for 3.4.0.