Update CentOS install guide to GitLab 7.3.
Minor ruby version typo.
Fix location of redis.conf
Add cmake as dependency. Needed for rugged.
Replace Debian/Ubuntu with CentOS.
Merge branch 'centos-7_3' into 'master'

Update CentOS6 install guide to GitLab 7.3

See merge request !27
Merge branch 'master' of gitlab.com:gitlab-org/gitlab-recipes