Update Mattermost to 3.7.3 for GitLab 9.0.1

Mattermost 3.7.3 available now from https://www.mattermost.com/download/

This dot release addresses a security issue that affects all previous versions, and includes a bug fix with telemetry data collection. Upgrading to Mattermost 3.7.3 is highly recommended.

You can visit https://gitlab.mattermost.com to verify and test the release. Full release notes can be found here. No changes are required for gitlab.rb.

Please let me know if you have any questions. Thanks!