Update mattermost.rb to 4.0.5 for GitLab 9.4

Open username-removed-570793 requested to merge jasonblais/omnibus-gitlab:patch-12 into 9-4-stable

This dot release addresses security issues in the 4.2.0 release that are backported in this 4.0.5 release.

Once you know which GitLab 9.4.X release this Mattermost version will be included, let me know and I can update the changelog.

Edited by username-removed-570793