Update GitLab Mattermost docs for MM v4.2, GitLab v10.0

Merged username-removed-570793 requested to merge jasonblais/omnibus-gitlab:patch-9 into master

Update GitLab Mattermost docs for MM v4.2, GitLab v10.0.

See PR that adds MM v4.2 to GitLab v10.0: https://gitlab.com/gitlab-org/omnibus-gitlab/merge_requests/1932