Rails 6 compatibility

Fixes

• Rails 6 compatibility. #36
• Explicit range matching. #33 #34
• Missing require 'chronic' for the default theme. #28

v1.0.6

• Fixes compatibility with the new Rails responders #19. Thanks, @Paxa!
• Adds a config options to set an alternate responder key, allowing for compatibility with axlsx-rails #21. Thanks, @mintuhouse!