Update install/CentOS_6.md

Updated gitlab-centos script to 4-0-stable to fix unicorn and sidekiq startup errors.
Merge pull request #1 from diegotdai/patch-1

Update install/CentOS_6.md
Merge branch 'master' of https://github.com/gitlabhq/gitlab-recipes
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>
Merge pull request #98 from nielsbasjes/CentOSGitlab51

Installing Gitlab 5.2 on CentOS 6.4