20200703113000: `require "celluloid/current"` is removed. use `require "celluloid"`

20200613113000: Remove $CELLULOID_DEBUG global variable. Fallback to [Logger](https://ruby-doc.org/stdlib-2.2.0/libdoc/logger/rdoc/Logger.html) mechanisms. [Issue #742](https://github.com/celluloid/celluloid/issues/742)
20200703113000: `require "celluloid/current"` is removed. use `require "celluloid"`

Add Refactor note (remove global variable)
Add Refactor note (remove global variable)
