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.

Not knowing the inner workings of Gitlab, I didn't realize my web server had to be setup and running first.

I've edited the installation guide to move the `gitlab:check` to after we will the web server, avoiding errors.