ProxyPreserveHost On: reason for apache2 to not be able to reload.

Hello! First of all, thank you for writing this .conf, it's a great time saver for a newcomer that's trying to setup a local Gitlab CE for a LAN!

Unfortunately, somewhere through my setup, I came with a problem, after enabling the .conf website (I have an Ubuntu 16.04 LTS machine as a home server) via a2ensite command, I then proceeded to reload apache2, with apache2 saying it couldn't because there was an error, so I checked what journalctl (-xe) reported and it said that there was an error in line 17 of what was written in the gitlab-omnibus-apache24.conf saying that proxyPreserveHost was invalid command.

I'm not really sure what to do from here on out as I just barely started figuring out finally how to do Virtualhosting so apache2 options are something I have no clue on where to go from here.

I'll await your answer for the meantime!

p.s.: the reason for my attempt at making a vhost in apache2 is because Gitlab CE tends to "take over" apache2 entirely so it prevents from me to even be able to access the test /var/www/html/ directory that apache2 has on default for Ubuntu 16.04 (that "It Works!" web page)

Anyway, thanks in advance.