Use Ruby 2.3.1

## What does this MR do?

This WIP MR will eventually introduce 2.3 as default language for GitLab CE/EE

## Why was this MR needed?

@connorshea wants to be prepared for Rails 5.0, so we need Ruby > 2.2, so why not go 2.3 already?

## What are the relevant issue numbers?

Closes #12507 

## TODO

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [x] All builds are passing
- [ ] [Omnibus uses 2.3.1](https://gitlab.com/gitlab-org/omnibus-gitlab/merge_requests/847)

See merge request !4948