Update doc/install/installation.md

Line 295: sudo /etc/init.d/gitlab restart
Results in: "Error, unicorn not running!"
This is because unicorn is not yet running for first boot. I suggest changing this to 'start'.

Line 305: sudo apt-get install nginx
Change command to: sudo apt-get -y install nginx
Allowing automated install to simplify and follow pattern of earlier installs.
Merge pull request #2723 from ZKjellberg/patch-1

Update doc/install/installation.md