[skip changelog] deprecate latest links (arduino#346)

* deprecate latest links

* Update README.md

Co-Authored-By: per1234 <accounts@perglass.com>

* rephrase

Upgrade protobuf to v3.9.1 on the CI

Now we can have the same version on both drone.io and AppVeyor, see PR arduino#329 for the details.

Add s3 upload support in .goreleaser.yml via `blob` configuration

- used blob configuration instead of deprecated s3 one. this requires
adding a AWS_DEFAULT_REGION env var to the drone step (toghether with
the IAM credentials), because this
property is missing in the blob config