Follow googlecartographer/cartographer#686. (#78)

cartographer_turtlebot/configuration_files/turtlebot_depth_camera_2d.lua

@@ -51,7 +51,7 @@ TRAJECTORY_BUILDER_2D.ceres_scan_matcher.rotation_weight = 300
 
 TRAJECTORY_BUILDER_2D.submaps.resolution = 0.035
 TRAJECTORY_BUILDER_2D.submaps.num_range_data = 120
-POSE_GRAPH.optimize_every_n_scans = 120
+POSE_GRAPH.optimize_every_n_nodes = 120
 POSE_GRAPH.constraint_builder.min_score = 0.82
 POSE_GRAPH.constraint_builder.sampling_ratio = 1.