Updating GitLab Mattermost to v1.4.0 final for GitLab 8.4 release

Hi @marin,

Final version of Mattermost v1.4.0 now ready for download at https://github.com/mattermost/platform/releases/tag/v1.4.0 as well as via wget https://github.com/mattermost/platform/releases/download/v1.4.0/mattermost.tar.gz

We've tested Mattermost v1.4.0 on https://gitlab.mattermost.com/community and it's running as expected.

Replacing the file should be only change needed. We're ready to test GitLab omnibus RC when ready.