Fix instance state checks to use constants instead of methods

Author: neddp

src/bosh-director/lib/bosh/director/deployment_plan/instance_plan.rb

@@ -193,8 +193,8 @@ def state_changed?
 
           return true if unresponsive_agent?
 
-          if (instance.state.stopped? && instance.current_job_state.running?) ||
-             (instance.state.started? && !instance.current_job_state.running?)
+          if (instance.state == Instance::STATE_STOPPED && instance.current_job_state.running?) ||
+             (instance.state == Instance::STATE_STARTED && !instance.current_job_state.running?)
             @logger.debug("Instance state is '#{instance.state}' and agent reports '#{instance.current_job_state}'")
             return true
           end