Require active_support/duration.

ActiveSupport::TimeWithZone references `ActiveSupport::Duration` but
does not require it, which can result in a `LoadError` when required
directly without requiring a component less granular like
`active_support/time`, where the autoload for `ActiveSupport::Duration`
is set up.

activesupport/lib/active_support/time_with_zone.rb

+require 'active_support/duration'
 require 'active_support/values/time_zone'
 require 'active_support/core_ext/object/acts_like'