check for existence of update hook

It seems that in the latest version of gitlab-shell they have removed the update hook:
https://gitlab.com/gitlab-org/gitlab-shell/commit/7eb45672b77a0337e6568ef64cd84e13027f2d7b

I'm using 
* gitlab_branch         => '7-4-stable',
* gitlabshell_branch    => 'v2.1.0',

I've adopted this solution because it seems to me the most compatible with different versions of gitlab and gitlab-shell.

Comments are welcome :+1: