Upgrade serialize signature in migration

Author: tf

db/migrate/20190531145431_insert_file_usages_for_scraped_sites.rb

@@ -6,7 +6,7 @@ class MigratedPage < ActiveRecord::Base
 
     belongs_to :chapter, class_name: 'MigratedChapter'
 
-    serialize :configuration, JSON
+    serialize :configuration, coder: JSON
 
     def configuration
       super || {}