Upgrade to Ruby 2.5.3

We found in CE that the main difference in behavior is with URI.join (https://gitlab.com/gitlab-org/gitlab-ce/issues/53180). No instances in gitaly-ruby were found of this call.

Ruby 2.5.x claims to be 5-10% faster than Ruby 2.4.x, so let's use it.

• CE MR: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22555
• EE MR: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8069
• omnibus-gitlab MR: omnibus-gitlab!2806 (merged)