Procfile: run etcd from bin/etcd

`./build` places the etcd binary in `bin/etcd`. Procfile should use that location.
...answered before you suggested to just use `go install`. I personally think `go install` is adding an unnecessary layer of indirection for the purpose of this script. I just want to build the binary and put it in `./bin/`. I don't know what `GOBIN=$(pwd)/bin go install` is getting us