Upgrade to Ruby 2.5.3

See https://www.ruby-lang.org/en/news/2017/12/25/ruby-2-5-0-released/

• EE MR: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8069
• Gitaly MR: gitaly!942 (merged)
• omnibus-gitlab MR: omnibus-gitlab!2806 (merged)
• GDK MR: gitlab-development-kit!550 (merged)

Attempt to update google-protobuf for migration-paths. Because the one we were using aren't compatible with Ruby 2.5.3, and it'll be troublesome to switch Ruby. Upgrading google-protobuf will be much easier.

All of them will need to be updated for Ruby 2.5.3.