go-tour: update loops solution with shorter version
go-tour: update loops solution with shorter version

Fixes issue 171.

LGTM=r
R=golang-codereviews, r
CC=golang-codereviews
https://codereview.appspot.com/79240043

Go-zh/tour-old,minux/go-tour-zh,minux/go-tour-zh,Go-zh/tour-old