Move Ubuntu 18.04 builds into earlier build stage
Move Ubuntu 18.04 builds into earlier build stage

GitLab.com will be updating the hosts to Ubuntu 18.04
(https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/6539), so
we should make sure these packages are available with Ubuntu 16.04.

gitlabhq/omnibus-gitlab,gitlabhq/omnibus-gitlab,gitlabhq/omnibus-gitlab,gitlabhq/omnibus-gitlab