[Guide] Gitlab 6.3 - CentOS 6.5 - Increment CentOS, ruby, gitlab-shell, and gitlab versions

Updated versions in the CentOS guide:

CentOS 6.4 -> 6.5
ruby-2.0.0-p247 -> p353
gitlab-shell 1.7.4 -> 1.8.0
gitlab 6.2 -> 6.3