fix Order#reload method signature

ActiveRecord::Base#reload accepts an `options` argument: https://github.com/rails/rails/blob/v4.1.4/activerecord/lib/active_record/persistence.rb#L386
This code was failing for me:

    >> Spree::Order.first.with_lock { }
    ArgumentError: wrong number of arguments (1 for 0)
    from /Users/jordan/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/bundler/gems/spree-287d40eaf236/core/app/models/spree/order.rb:592:in `reload'
looks like it's just for clearing the tax zone right? it does seem reasonable that you might want to reset state like the tax_zone when reloading the order yeah?