Update gitlab & gitlab-shell
readme: configure the environment in the usage example

If the environment is not set, then `gitlab['shell_path']` will be nil and result in an error during the chef run. Setting the environment fixes this issue.

Signed-off-by: Dominik Richter <dominik.richter@gmail.com>
Merge pull request #22 from g3d/bugfix/critical-vulnerabilities

Multiple Critical Vulnerabilities in GitLab
Merge pull request #23 from arlimus/master

readme: configure the environment in the usage example