URDF files have been edited to offer ros-melodic support
original repository: https://github.com/mas-group/youbot_simulation & https://github.com/mas-group/youbot_description
Files in youbot_description has been modified to work on ros-melodic.
In case of RLException: Invalid tag: Cannot load command parameter robot_description or 'arm_1_ref' link error, update the xacro package:
sudo apt-get install ros-melodic-xacro
roslaunch youbot_gazebo_robot youbot.launch