Add gitlab-grit gem

Not 100% sure if this is correct. RubyGems https://rubygems.org/gems/gitlab-grit shows the new version 2.6.1. Within the original Repository https://github.com/gitlabhq/grit i didn't see any entry. Within this commit https://github.com/gitlabhq/gitlabhq/commit/1040e88e5645fc3dccf55b418bfbf04d307f3d8b one can see that http://gitlab.org updated the gitlab_git gem which depends on the new gitlab-grit gem for security reasons.