Touch page version's updated at when updating

Without this, the previous commit will not do what we expect. If we
don´t update the page version's updated at, it will not serve as a good
cache invalidator.

app/models/alchemy/page/publisher.rb

@@ -35,6 +35,7 @@ def publish!(public_on:)
                 end
               end
             end
+            version.update(updated_at: public_on)
             page.update(published_at: public_on)
           end
         end

spec/models/alchemy/page/publisher_spec.rb

@@ -55,8 +55,9 @@
     end
 
     context "with published version existing" do
+      let(:yesterday) { Date.yesterday.to_time }
       let!(:public_version) do
-        create(:alchemy_page_version, :with_elements, element_count: 3, public_on: Date.yesterday.to_time, page: page)
+        create(:alchemy_page_version, :with_elements, element_count: 3, updated_at: yesterday, public_on: yesterday, page: page)
       end
 
       let!(:nested_element) do
@@ -67,6 +68,10 @@
         expect { publish }.to_not change(page.public_version, :public_on)
       end
 
+      it "updates public version's updated_at timestamp" do
+        expect { publish }.to change(page.public_version, :updated_at)
+      end
+
       it "does not create another public version" do
         expect { publish }.to_not change(page.versions, :count)
       end

spec/models/alchemy/page_spec.rb

@@ -711,13 +711,13 @@ module Alchemy
       end
 
       around do |example|
-        travel_to(Time.parse("2019-01-01 12:00:00")) do
+        travel_to(Time.parse("2019-01-01 12:00:00 UTC")) do
           example.run
         end
       end
 
       it "returns a cache version string" do
-        expect(page.cache_version).to eq("2018-12-31 11:00:00 UTC")
+        expect(page.cache_version).to eq("2018-12-31 12:00:00 UTC")
       end
     end