nacl.cr.yp.to is available over HTTPS and presents a valid certificate, so seems useful to use it. Change-Id: I64d86e52cc314641b4169167fe5123d38c2904c0 Reviewed-on: https://go-review.googlesource.com/26812 Reviewed-by: Jonathan Rudenberg <jonathan@titanous.com> Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>