dashboard: update plan9 VM image

This version adds a TCP "close" message:

    http://9legacy.org/9legacy/patch/9-tcp-close.diff

Updates golang/go#15464

Change-Id: I4866858338db4b408fd180b5d89560aa86049adc
Reviewed-on: https://go-review.googlesource.com/31327
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/dashboard/builders.go b/dashboard/builders.go
index 07bfbac..1708cd7 100644
--- a/dashboard/builders.go
+++ b/dashboard/builders.go
@@ -100,7 +100,7 @@
 		goBootstrapURLTmpl: "https://storage.googleapis.com/$BUCKET/gobootstrap-netbsd-amd64.tar.gz",
 	},
 	"host-plan9-386-gce": &HostConfig{
-		VMImage:            "plan9-386-v3",
+		VMImage:            "plan9-386-v4",
 		Notes:              "Plan 9 from 0intro; GCE VM is built from script in build/env/plan9-386",
 		buildletURLTmpl:    "http://storage.googleapis.com/$BUCKET/buildlet.plan9-386",
 		goBootstrapURLTmpl: "https://storage.googleapis.com/$BUCKET/gobootstrap-plan9-386.tar.gz",