This updates the default Directory URL to Let's Encrypt v2. Their v1 has been deprecated and new account registrations will be disabled in Nov 2019. See the following for details: https://community.letsencrypt.org/t/end-of-life-plan-for-acmev1/88430 There's no good reason to keep the default URL pointing to v1 any longer. Updates golang/go#21081 Change-Id: I34520e4dfb04a30d82a50559369bac819076d49c Reviewed-on: https://go-review.googlesource.com/c/crypto/+/200638 Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>