Upgrade to Ruby 2.5

Summary:
As the dependencies have been updated in 59f60a4160d7,
the repository is ready to be used in Ruby 2.5.1.

Use the more universal .ruby files instead of .rvmrc,
so it's compatible with other Ruby managers too.

Fixes T1446.

Test Plan: ruby tommy.rb -o 0.0.0.0

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1446

Differential Revision: https://devcentral.nasqueron.org/D1780