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 (closed)

TODO

• CHANGELOG entry added
• Documentation created/updated
• All builds are passing
• Omnibus uses 2.3.1