http2/h2demo: require golang.org/x/net@latest

Now that a pseudo-version of golang.org/x/net with h2demo carved exists,
we can depend on it. Add the dependency to h2demo's go.mod file.

Updates golang/go#30685

Change-Id: I3dbb2493d97be381350881228025d27c7e8e8623
Reviewed-on: https://go-review.googlesource.com/c/net/+/166857
Run-TryBot: Michael Matloob <matloob@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

http2/h2demo/go.mod

@@ -1,15 +1,11 @@
 module golang.org/x/net/http2/h2demo
 
-// TODO: Require a future pseudo-version of golang.org/x/net module
-//       where h2demo has been carved out.
-//       Can't do that now because that pseudo-version doesn't exist yet;
-//       it's being created in this very change.
-
 require (
 	cloud.google.com/go v0.36.0
 	go4.org v0.0.0-20190218023631-ce4c26f7be8e
 	golang.org/x/build v0.0.0-20190311051652-b8db43dd7225
 	golang.org/x/crypto v0.0.0-20190308221718-c2843e01d9a2
+	golang.org/x/net v0.0.0-20190311175648-12eef18f757f
 )
 
 replace golang.org/x/net => ../..