Use page version's updated_at timestamp as cache key

Currently, it's hard for us to invalidate caches for Alchemy pages when
content that's referenced through ingredients with related objects
changes.

For example, in a situation where a user combines Alchemy and Solidus
using the `alchemy_solidus` gem, a page's cache key does not update when
a product that's referenced through a SpreeProduct ingredient changes.

There's a PR up on the alchemy_solidus gem that touches ingredients in
these situations, and with this change, that touching can be used for
breaking caches. [1]

[1](AlchemyCMS/alchemy-solidus#108)

app/models/alchemy/page/page_natures.rb

@@ -103,15 +103,13 @@ def layout_partial_name
 
       # Returns the version that's taken for Rails' recycable cache key.
       #
-      # Uses the +published_at+ value that's updated when the user publishes the page.
-      #
-      # If the page is the current preview it uses the +updated_at+ value as cache key.
+      # Always uses the +updated_at+ value.
       #
       def cache_version
         if Current.preview_page == self
-          updated_at.to_s
+          draft_version.updated_at.to_s
         else
-          published_at.to_s
+          public_version.updated_at.to_s
         end
       end
 

spec/models/alchemy/page_spec.rb

@@ -708,9 +708,12 @@ module Alchemy
       let(:last_week) { Time.current - 1.week }
 
       let(:page) do
-        build_stubbed(:alchemy_page, updated_at: now, published_at: last_week)
+        build_stubbed(:alchemy_page, public_version: public_version, draft_version: draft_version)
       end
 
+      let(:public_version) { build_stubbed(:alchemy_page_version, updated_at: last_week) }
+      let(:draft_version) { build_stubbed(:alchemy_page_version, updated_at: now) }
+
       subject { page.cache_version }
 
       before do