Fix departed id counting (#704)

Author: eugenevinitsky

flow/core/kernel/vehicle/traci.py

@@ -191,7 +191,7 @@ def update(self, reset):
             self._num_departed.append(
                 len(sim_obs[tc.VAR_DEPARTED_VEHICLES_IDS]))
             self._num_arrived.append(len(sim_obs[tc.VAR_ARRIVED_VEHICLES_IDS]))
-            self._departed_ids.append(sim_obs[tc.VAR_ARRIVED_VEHICLES_IDS])
+            self._departed_ids.append(sim_obs[tc.VAR_DEPARTED_VEHICLES_IDS])
             self._arrived_ids.append(sim_obs[tc.VAR_ARRIVED_VEHICLES_IDS])
 
         # update the "headway", "leader", and "follower" variables