Support for Ruby 2.5

Description

GitLab currently requires Ruby 2.3 which (given the rhythm of the Ruby release calendar) will most likely enter security maintenance mode by the end of March 2018 and EOL 12 months later.

Switching to Ruby 2.4 might be quite painless since a number of installations are running GitLab on Ruby 2.4 for quite some time already.

Ruby 2.5 on the other hand might cause more work. As an example, the required version 1.4.3 of grpc is not compatible with Ruby 2.5 (however, newer versions are).

Proposal

Investigate updating GitLab to support Ruby 2.5.

Links / references

omnibus-gitlab#2588 (closed)

/label ~"feature proposal" ruby

Relevant merge requests

CE

1. https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22555
2. https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22951
3. https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22948
4. https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22954

EE

1. https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8069
2. https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8084
3. https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8387

Gitaly

1. gitaly!942 (merged)

omnibus-gitlab

1. omnibus-gitlab!2806 (merged)