Upgrade acme-client to v2.0.5

Let's Encrypt dropped support of unauthenticated GET requests to their
staging API. This version uses POST-as-GET requests, which is the
recommended method.

This mirrors the change in Omnibus:
https://gitlab.com/gitlab-org/omnibus-gitlab/merge_requests/3782

Full CHANGELOG:
https://github.com/unixcharles/acme-client/blob/master/CHANGELOG.md

Diff:
https://github.com/unixcharles/acme-client/compare/v2.0.2..v2.0.5