upstream urdf/model.h broke a line in drivers

[moriarty]

There is a one line fix which needs to go into our private fetch_drivers code.

I locally patched around in the meantime, and almost forgot that I had.

I'm opening the ticket here in public to track it with the other 18.04 & Melodic issues:
The line isn't very secret:

-  boost::shared_ptr<const urdf::Joint> urdf_joint = model.getJoint(joint->getName());
+  std::shared_ptr<const urdf::Joint> urdf_joint = model.getJoint(joint->getName());

A backwards compatible patch for both Indigo & Melodic is possible, but then we need to ensure those customers running Indigo have at least this PR:
ros/robot_model#182 ( repo has since been broken up -> https://github.com/ros/urdf )