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I don't think that you're publishing to the correct topic (unless that topic is being remaped). From a quick search in the GitHub repo it looks like you should be publishing to the cmd_vel topic or some variant of it. The /mobile_base/commands/velocity topic looks like it's used for muxing the navigation commands. If you really are supposed to be publishing to the /mobile_base/commands/velocity topic then please update your question with the results of rostopic list.

As a side note, please take a look at the environment variables page on the wiki. If you're going to be using an IP instead of a host name then you should use ROS_IP not ROS_HOSTNAME. I've also never seen anyone use

sudo sh -c 'ROS_HOSTNAME=192.168.3.139 >> /opt/ros/indigo/setup.bash'

before.