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Cosmic is officially out-of-date, but since it still may be in use, and because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

contrib/packaging/Makefile

@@ -6,10 +6,10 b' DEBIAN_CODENAMES := \\'
   buster
 
 UBUNTU_CODENAMES := \
-  trusty \
   xenial \
-  artful \
   bionic \
+  cosmic \
+  disco
 
 FEDORA_RELEASES := \
   20 \