cmd/godoc: add version info for golang.org

Adds version information for package docs for the production version of
godoc running on golang.org.

Updates golang/go#5778.

Change-Id: I8b56e8152b20b34104f274263a6c0b5a0180093b
Reviewed-on: https://go-review.googlesource.com/c/139557
Reviewed-by: Devon H. O'Dell <devon.odell@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

cmd/godoc/appinit.go

@@ -83,6 +83,7 @@ func main() {
 		log.Fatal(err)
 	}
 	corpus.IndexDirectory = indexDirectoryDefault
+	corpus.InitVersionInfo()
 	go corpus.RunIndexer()
 
 	pres = godoc.NewPresentation(corpus)

cmd/godoc/regtest_test.go

@@ -49,6 +49,11 @@ func TestLiveServer(t *testing.T) {
 			Path:      "/pkg/os/",
 			Substring: "func Open",
 		},
+		{
+			Path:      "/pkg/net/http/",
+			Substring: `title="Added in Go 1.11"`,
+			Message:   "version information not present - failed InitVersionInfo?",
+		},
 		{
 			Path:        "/robots.txt",
 			Substring:   "Disallow: /search",