CentOS guide intro incompatible gitlab and gitlab-shell combination

[xilin]

According to this issue https://gitlab.com/gitlab-org/gitlab-ce/issues/700, 7.4 is incompatible with 2.1.0

[axilleas]

Yeah, the guide needs updating :/ You can submit a PR if you like otherwise I'll do it whenever I find some free time.

[xilin]

@axilleas I've created a PR.