Add timezone configuration to gitlab.yml

This pull request refer to this suggestion: http://feedback.gitlab.com/forums/176466-general/suggestions/5890076-add-timezone-configuration-to-gitlab-yml

It's now possible to set time zone form gitlab.yml configuration file by adding a little time_zone initializer.
Because user's are using it actually. So keeping it will avoid BC breaks and the note will suggest to move to gitlab.yml file. In my opinion. ;)