2017/state-of-go-may: add +build go1.9 constraint for Go 1.9+ samples

Fixes golang/go#20424.

Change-Id: I4e4721d1f61696b36ebe418457c3c51afbf0c1a6
Reviewed-on: https://go-review.googlesource.com/43731
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
3 files changed