Upgrade Ruby version to 2.5

Summary:
This is a follow-up for dee8c80a7244.

Ref T1446

Test Plan: Build image, run it, browse https://builds.nasqueron.org/

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1446

Differential Revision: https://devcentral.nasqueron.org/D1783