For those who were still having problems getting Mattermost running on CentOS 7, I was also having the same problems and couldn't get beyond the postgres install. I found another guide and with a few tweaks, I was able to get this running.
https://www.howtoforge.com/tutorial/install-mattermost-with-postgresql-and-nginx-on-centos7/
The change that was required from this guide was in the "Download and Extract Mattermost"
Instead of...
[root@mattermost ~]# wget -q "https://github.com/mattermost/platform/releases/download/v2.0.0/mattermost.tar.gz" -O mattermost.tar.gz
Use this...
wget https://releases.mattermost.com/3.3.0/mattermost-team-3.3.0-linux-amd64.tar.gz
Then in the next block, instead of...
tar -xvzf mattermost.tar.gz
Use this...
tar -xvzf mattermost-team-3.3.0-linux-amd64.tar.gz
After that, follow the guide to the letter and it will get you to this...
0_1473265786807_upload-d89fe07b-7d71-4d39-8dd7-e00395f6aa84
@aaronxiang @wirestyle22 @scottalanmiller