Fix deprecation warning for Spree::Shipment#reverse_chronological scope

[aitbw]

Hi everyone! 👋

This is a small PR to fix a deprecation warning that shows up when running certain tests. The message goes like this:

DEPRECATION WARNING: Dangerous query method (method whose arguments are used as raw
SQL) called with non-attribute argument(s): "coalesce(spree_shipments.shipped_at,
spree_shipments.created_at) desc". Non-attribute arguments will be disallowed in Rails 6.0.
This method should not be called with user-provided values, such as request parameters or
model attributes. Known-safe values can be passed by wrapping them in Arel.sql().
(called from block in <class::Shipment> at /solidus/core/app/models/spree/shipment.rb:36)

From what I can see, the attributes used in here shouldn't generate any problems as they just store timestamps and the user is not passing any parameters to said query so SQL injection is unlikely.

Running the full test suite following this patch doesn't show the aforementioned warning anymore. This PR also future proofs this scope for an eventual Rails 6 update.

Let me know what you think!

[kennyadsl]

Thanks! Just left a (non-blocking) possible improvement, let me know if you think it makes sense to do that here

[jacobherrington]

This makes sense to me, good catch @aitbw!