Update payment_decorator.rb

was getting hit this this error upon "rails g spree_subscriptions:install":

/Users/ethanczahor/.rvm/gems/ruby-2.0.0-p0/gems/state_machine-1.2.0/lib/state_machine/machine.rb:2260:in `block in add_states': "completed" state defined as String, :checkout defined as Symbol; all states must be consistent (ArgumentError)