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#31567

Change-Id: I2d779a143c711e86fb2f34451d7398d3514623ae
Reviewed-on: https://go-review.googlesource.com/c/website/+/192559
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alexander Rakoczy <alex@golang.org>

go.mod

@@ -1,5 +1,7 @@
 module golang.org/x/website
 
+go 1.11
+
 require (
 	cloud.google.com/go v0.34.0
 	github.com/gomodule/redigo v2.0.0+incompatible