Removes use of class variable to share Sync

Perhaps I'm being pedantic, but I always see the use of a class variables as a red flag.

If a class that inherits from FinteMachine::StateMachine modifies @@sync, every FiniteMachine::StateMachine will be affected by that change.

     class FiniteMachine::StateMachine
       def current_sync
          @@sync
       end
     end

     state_machine = FiniteMachine::StateMachine.new

     state_machine.current_sync -----> Instance of Sync

     class OtherStateMachine < FiniteMachine::StateMachine
        @@sync = something_else
     end

     state_machine.current_sync -----> something_else