Update the content schema

The content schemas were modified in PR: alphagov/govuk-content-schemas#374

The content published by the calculators app has to conform to the updated
schema.

app/presenters/calculator_content_item.rb

@@ -22,7 +22,8 @@ def payload
     {
       title: calculator.title,
       base_path: base_path,
-      format: 'placeholder_calculator',
+      schema_name: 'placeholder_calculator',
+      document_type: 'calculator_document',
       details: {},
       publishing_app: 'calculators',
       rendering_app: 'calculators',