fix frame_id related problem

After applying PR #8, `frame_id` related problem still happened. 

~~~
average rate: 29.855
	min: 0.013s max: 0.040s std dev: 0.00295s window: 110
stereo_image_proc: /tmp/buildd/ros-hydro-image-geometry-1.10.18-0precise-20140819-2049/src/stereo_camera_model.cpp:35: bool image_geometry::StereoCameraModel::fromCameraInfo(const CameraInfo&, const CameraInfo&): Assertion `left_.tfFrame() == right_.tfFrame()' failed.
[stereo_image_proc-9] process has died [pid 24876, exit code -6, cmd /opt/ros/hydro/lib/stereo_image_proc/stereo_image_proc _approximate_sync:=True __name:=stereo_image_proc __log:=/home/yutaka/.ros/log/fb9ff4ba-422d-11e4-b275-00031d0df531/stereo_image_proc-9.log].
log file: /home/yutaka/.ros/log/fb9ff4ba-422d-11e4-b275-00031d0df531/stereo_image_proc-9*.log
average rate: 29.745
	min: 0.018s max: 0.043s std dev: 0.00322s window: 140
~~~

So I fix it with this PR. Please check and merge it.