Reorder steps to install web server before running gitlab:check

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.