Update GitLab Mattermost docs for MM v4.2, GitLab v10.0
Update GitLab Mattermost docs for MM v4.2, GitLab v10.0.
See PR that adds MM v4.2 to GitLab v10.0: !1932 (merged)
Update GitLab Mattermost docs for MM v4.2, GitLab v10.0.
See PR that adds MM v4.2 to GitLab v10.0: !1932 (merged)