acme: specify which version exactly is implemented

The WG has recently published v3 of the spec.
The package implementation is still at v2. So is Let's Encrypt, to be
honest.

Regardless, I think it makes sense to point to the exact version
the package is compatible with.

Change-Id: Ibb3fc51aae363d72f67f7e876ef46c4f51c8da0b
Reviewed-on: https://go-review.googlesource.com/25451
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>