Update Order Sync

Orders can not be synced in the same way that they were in the older
API. If there is no order to remove, Mailchimp will raise an error.
Instead rely on adding an order (since this will be done most often) and
if there is a slim chance that an existing order is already in the
  system (we are syncing an order), then remove it and add the new one.
Merge branch 'feature/mailchimp-2.0' of github.com:DynamoMTL/spree_chimpy into feature/mailchimp-2.0