Support `ActiveSupport::TimeZone` as `Chronic.time_class` (fixes #208)

Finally done :) It took way more time than should. For some reason `is_a?`, `kind_of?`, `instance_of?` didn't worked in tests, but did work if I just run manually, seems `MiniTest` does some bad monkey patching or something else overrides something. So I'm using `time_class.name` to determine class.

Also I commented out ActiveSupport test because it requires `MiniTest ~> 4.2` but we're using `~> 5.0`. Have to wait till they update and then can comment out.