use churn rather than uproxypeerconnection

This integrates the SOCKS server with churn. Pretty simple, thanks to the identical interfaces. Depends on:
https://github.com/uProxy/uproxy-churn/pull/40

I wish there were a simple way to plug in different peerconnection providers but that seems like something to address at some future point.
Thanks Matt! I fixed that description, re-published `uproxy-churn`, and now see that Travis built successfully.