Default Spree::User from spree_auth_devise does not have first_name and last_name

The spree_auth_devise extension just uses `email` for its user display, should probably do the same in here.  See create table here: https://github.com/spree/spree_auth_devise/blob/master/db/migrate/20101026184949_create_users.rb or use of email display here: https://github.com/spree/spree_auth_devise/blob/master/lib/views/backend/spree/layouts/admin/_login_nav.html.erb#L3