go.mod: add go version directive

(I mostly just wanted to make a trivial change to this repo to clear
out the failure cells for misconfigured builders on the landing page
of https://build.golang.org.)

Updates golang/go#32836

Change-Id: Ibcea7e75912ca377fd78cef37d8beb29ab8b83d3
Reviewed-on: https://go-review.googlesource.com/c/review/+/192557
Run-TryBot: Bryan C. Mills <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>

Author: Bryan C. Mills

go.mod

@@ -1 +1,3 @@
 module golang.org/x/review
+
+go 1.11