Make gci-proxy work with unix sockets

[danielfireman]

Apparently, they are faster: https://stackoverflow.com/questions/14973942/tcp-loopback-connection-vs-unix-domain-socket-performance

[danielfireman]

Also, puma has built in support for it: https://github.com/puma/puma/tree/c2d1577feae2aa0c351a1dc1fe6e679bd900541b#binding-tcp--sockets