Reorder steps to install web server before running gitlab:check

Created by: sajanp

I was following the install guide for a fresh install of Gitlab 6.2 and got the point where we "Double Check" everything using gitlab:check. I kept getting an error saying gitlab-shell couldn't start, with no apparent reason.

gitlab-shell cannot start without a running web server, however the installation guide does not instruct to install the webserver until after running gitlab:check.

I've updated the docs to run gitlab:check after the web server is installed and running.