This reverts commit b0e6bc3ffd02146fd999f12975f269dac94a0994.

Reinstate the updated `TimeDuration` spec code. Seems like this was
reverted by accident. Robert was doing a complicated merge/rebase and
somehow in the mix this commit ended up being unintentionally reverted.

> 16:23:26 +0200 < robert> @teddy merge idiosynchrasy I did not touch these specs deliberately but must have pulled in the old version by error sorry
> 16:24:19 +0200 < robert> I had some strange conflicts and rebased twice, hmmm, strange though I just did not touch these specs for months
> ...
> 16:25:10 +0200 < robert> anywyay feelfree to undo my undoing :blush:

This reverts commit 52d2c968cac7e901b8e6fa3d1be115239b7c5937.

Change the variable names to `earlier` and `later`, and use explicit
`true` and `false` values to be consistent with the newer tests in this
file. The new variable names in particular should be a little quicker to
understand.

Refs #870

Previously, the `#exceeds_gap?` method would modulo the
`later - earlier` duration against 24 hours such that no duration
exceeded 24 hours, and the resulting duration being compared was
adjusted to fit inside a single day.

When we removed the numbers from `exceeds_gap?`, we also removed the
modulo operation. I had a feeling that would potentially cause problems,
and it turns out it did. Though it still took too much investigation
time to realise that the most obvious answer was in fact the correct
one. Blame the confusing test failures if I will:

    Failures:

      1) Chouette::VehicleJourney state_update should return errors when validation failed
         Failure/Error: expect(state['errors'][:vehicle_journey_at_stops].size).to eq 1

         NoMethodError:
           undefined method `[]' for nil:NilClass
         # ./spec/models/chouette/vehicle_journey_spec.rb:148:in `block (3 levels) in <top (required)>'

      2) Chouette::VehicleJourney state_update vehicle_journey_at_stops should return errors when validation failed
         Failure/Error: expect(item['errors'][:arrival_time].size).to eq 1

         NoMethodError:
           undefined method `[]' for nil:NilClass
         # ./spec/models/chouette/vehicle_journey_spec.rb:189:in `block (4 levels) in <top (required)>'

      3) VehicleJourneyImport.save should not import vehicle_journeys and not create objects when vehicle journey at stops are not in ascendant order
         Failure/Error: expect(VehicleJourneyImport.new(:route => route, :file => invalid_file_on_vjas_object).save).to be_falsey

           expected: falsey value
                got: true
         # ./spec/models/vehicle_journey_import_spec.rb:90:in `block (3 levels) in <top (required)>'

    Finished in 4 minutes 20.7 seconds (files took 14.47 seconds to load)
    1089 examples, 3 failures, 23 pending

    Failed examples:

    rspec ./spec/models/chouette/vehicle_journey_spec.rb:137 # Chouette::VehicleJourney state_update should return errors when validation failed
    rspec ./spec/models/chouette/vehicle_journey_spec.rb:183 # Chouette::VehicleJourney state_update vehicle_journey_at_stops should return errors when validation failed
    rspec ./spec/models/vehicle_journey_import_spec.rb:89 # VehicleJourneyImport.save should not import vehicle_journeys and not create objects when vehicle journey at stops are not in ascendant order

These three tests failed with my version of `exceeds_gap?`.

To correct the failures, add back in the 24-hour adjustment. Put it in a
named method that is hopefully clear enough to understand.

The second test failure ("vehicle_journey_at_stops should return errors
when validation failed") used two times that were the same. I decided to
add a test for this because it seemed like an interesting edge case. The
test I added didn't fail even for the previous version of
`.exceeds_gap?`, but I figure it seems like a useful test anyway.

Refs #870

These are holdovers from when these specs were copied from
`vehicle_journey_at_stop_spec.rb`. They're not used in our specs now so
get rid of them.

Refs #870

Take the specs out of the `IncreasingTimeOrderValidator` spec and move
them into `TimeDuration`. This is part of the consolidation and complete
move of the `exceeds_gap?` method into `TimeDuration`.

Add a context to the two test blocks that specifies the duration at a 4
hour gap. Currently the implementation doesn't do anything with that
argument, but this makes it clearer that we're using a four hour
duration.

Refs #870