[x/tour] go-tour: various updates for Go 1
[x/tour] go-tour: various updates for Go 1

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/5702045
X-Tour-Commit: 5b795fb97d96203cb16b61371e404f87930b22ff

golang/website,golang/website,golang/website,golang/website,golang/website,golang/website,golang/website