Upgrade OkHttp to 2.0

Uses "the familiar java.net.HttpURLConnection API" in OkHttp 2.0 (new OkUrlFactory(client).open(url) vs. client.open(url)) and OkHttp's Cache instead of ResponseCache. 

This does not make the changes suggested in #392 (using the new OkHttp request/response style, or revisiting caching). This is primarily just a short term fix to upgrade Mapbox Android to work in projects that are already using OkHttp 2.0.