Add the dependency ceres-solver in the .rosinstall file. (#46)

cartographer_turtlebot.rosinstall

@@ -1,3 +1,4 @@
 - git: {local-name: cartographer, uri: 'https://github.com/googlecartographer/cartographer.git'}
 - git: {local-name: cartographer_ros, uri: 'https://github.com/googlecartographer/cartographer_ros.git'}
 - git: {local-name: cartographer_turtlebot, uri: 'https://github.com/googlecartographer/cartographer_turtlebot.git'}
+- git: {local-name: ceres-solver, uri: 'https://ceres-solver.googlesource.com/ceres-solver.git', version: '1.12.0rc4'}