Rails 6 Support

[stex]

This ticket is currently WIP regarding the necessary changes.

Discovered:

changed_attributes changes its behaviour

DEPRECATION WARNING: The behavior of changed_attributes inside of after callbacks will be changing in the next version of Rails. The new return value will reflect the behavior of calling the method after save returned (e.g. the opposite of what it returns now). To maintain the current behavior, use saved_changes.transform_values(&:first) instead. (called from changed_attributes at /Users/stex/workspace/gems/setting_accessors/lib/setting_accessors/integration.rb:73)