Add Rails 6.1 support

[timrwilliams]

Adds Rails 6.1 compatibility by updating the dummy app test.rb file to explicitly require active_support/core_ext/integer/time as per rails app:update command.

Details of the core Rails change can be found here: rails/rails#37391

All 82 specs pass running bundle exec rspec

[geeksam]

Thanks! Saw this earlier today but had to deal with a few urgent things at work. I'll check it out next week. :)

[geeksam]

New release pushed to rubygems.org. Thanks again, @timrwilliams!