Encode records with special chars as well

Using Marshal.dump on the French production database raised an error:

    Encoding::UndefinedConversionError: "\xC3" from ASCII-8BIT to UTF-8

Replacing Marshal with YAML solves the problem. It is also more reliable
and human readable.

This code was run against the French, Australian and UK production
data successfully.

db/migrate/20181123012635_associate_customers_to_users.rb

@@ -29,11 +29,11 @@ class AssociateCustomersToUsers < ActiveRecord::Migration
       joins("INNER JOIN spree_users ON customers.email = spree_users.email").
       where(user_id: nil).all
 
-    File.write(backup_file, Marshal.dump(customers))
+    File.write(backup_file, YAML.dump(customers))
   end
 
   def backed_up_customers
-    Marshal.load(File.read(backup_file))
+    YAML.load(File.read(backup_file))
   end
 
   def backup_file