Update Centos guide

Centos guide now follows GitLab official guide. I tested all the steps in a VM and was up and running. Additionally all links point to gitlab.com.

Closes #2 (closed) #3 (closed) #4 (closed)

Also waiting for https://github.com/gitlabhq/gitlab-recipes/pull/180 to get merged for the init script to play nice.

\cc @sytses