go.talks/present: make js use correct websocket address

This makes the client connect to the host from which it's being served,
rather than being hardcoded to localhost:3999.

Right now, if you try to run with -http set to any port other than
3999, you'll get websocket closed as soon as you open the presentation.

R=golang-dev
CC=adg, golang-dev
https://golang.org/cl/7829046
1 file changed