Update to OkHttp 2.0.0, which has a new OkUrlFactory

OkHttp changed API with v2.0.0, and the `client.open(url)` method no longer exists:

"URLConnection support has moved to the okhttp-urlconnection module. If you're upgrading from 1.x, this change will impact you. You will need to add the okhttp-urlconnection module to your project and use
the OkUrlFactory to create new instances of HttpURLConnection"

https://github.com/square/okhttp/blob/master/CHANGELOG.md#version-200-rc1