packaging: add Cosmic and Disco, remove Trusty and Artful - Trusty was publicly supported until 2019-04-30 - Artful was publicly supported until 2018-07-19 - Cosmic was publicly supported until 2019-07-18 - Disco will be publicly supported until 2020-01 Cosmic is officially out-of-date, but since it still may be in use, and because we didn't add it when it first came out, I think it would be nice to support it until the next time somebody decides to update this list of Ubuntu releases. Differential Revision: https://phab.mercurial-scm.org/D6761