Release 1.2.0

Changelog :

• Test with Ruby 2.4/2.5/2.6/2.7
• Test with Rails 5.x/6.x
• Switch to Zeitwerk to load gem files
• Add magic comment # frozen_string_literal: true
• Add binstubs to ease development
• Improve process name
• Improve systemd support
• Add Rubocop

Notes :

This is the last version to support Ruby 2.4.x
This is the last version to support Rails 5.0.x