Make process manager state reader strict

Author: andrzejkrzywda

contrib/ruby_event_store-process_manager/lib/ruby_event_store/process_manager.rb

@@ -67,7 +67,7 @@ def self.with_state(&state_class_block)
         end
 
         define_method(:state) do
-          @state ||= initial_state
+          @state
         end
 
         def self.included(host_class)