http2/h2demo: use Let's Encrypt for the TLS cert

The Google-issue one expired and it's too onerous to request another
and do it by hand.

Change-Id: I221406bc66de4b027b979fd87f6ec38f66318f9e
Reviewed-on: https://go-review.googlesource.com/33230
Reviewed-by: Tom Bergan <tombergan@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>