vehicle_journey_at_stops_day_offset_spec.rb: Use `build_stubbed`

I had assumed `build` wouldn't create objects in the database but it
does. Turns out what I actually wanted was FactoryGirl's `build_stubbed`
method, which I didn't know about before.

Thanks to:

http://myprogrammingblog.com/2015/12/21/10-ways-how-to-speed-your-unit-tests-rails-rspec-capybara/
https://robots.thoughtbot.com/use-factory-girls-build-stubbed-for-a-faster-test

for tipping me off about this.

Here's the time differential running this spec before and after the
change:

    Before:

    rspec spec/models/chouette/vehicle_journey_at_stops_day_offset_spec.rb
    Running via Spring preloader in process 39491
    Run options:
      include {:wip=>true}
      exclude {:js=>true, :meta=>true}

    All examples were filtered out; ignoring {:wip=>true}
    ...

    Finished in 3.98 seconds (files took 0.80415 seconds to load)
    3 examples, 0 failures

    ---

    After:

    rspec spec/models/chouette/vehicle_journey_at_stops_day_offset_spec.rb
    Running via Spring preloader in process 39511
    Run options:
      include {:wip=>true}
      exclude {:js=>true, :meta=>true}

    All examples were filtered out; ignoring {:wip=>true}
    ...

    Finished in 1.74 seconds (files took 0.73648 seconds to load)
    3 examples, 0 failures

Refs #3596

0 files changed, 0 insertions, 0 deletions