Update Sparkle to v2.0 from official repo

Previously, we forked the repo to the WriteFreely org because Sparkle didn't support tag-based SPM updates. We therefore had to pull from the master branch, so forking insulated us from unexpected changes.

As of 2.x Sparkle now allows for tag-based updating via SPM; we'll allow patch updates but stop short of 2.1 until we can vet changes.