Update install/CentOS_6.md

Updated gitlab-centos script to 4-0-stable to fix unicorn and sidekiq startup errors.
Fixed the centos init.d script for use with puma.
First cut of the updated installation guid for Gitlab 5.1 on CentOS 6.3
Minor textual improvements
Minor cleanups of the CentOS_6 install guide
Updated the installation guide to Gitlab 5.2

Signed-off-by: Niels Basjes <github@basjes.nl>
Do not duplicate files from the main repo.
Remove outdated scripts.
change