Upgrade versions

This PR updates Netty, protobuf and gson to the latest versions.

I tried upgrading Netty to 3.9.x before (last time was a few months back though). While it compiled fine, the client crashed when joining a network game. This does no longer happen for me today with 3.9.5 (tested three times). I *assume* that this was fixed either by the Netty folks or recent changes in engine.