Use new urdf SharedPtr typedefs

[nxdefiant]

Urdf has moved to a new SharedPtr type which results in a compile error:

/opt/ros/ros_catkin_ws/src/srdfdom/src/model.cpp: In member function 'void srdf::Model::loadGroups(const urdf::ModelInterface&, TiXmlElement*)':
/opt/ros/ros_catkin_ws/src/srdfdom/src/model.cpp:182:43: error: variable 'boost::shared_ptr<const urdf::Link> l' has initializer but incomplete type
       boost::shared_ptr<const urdf::Link> l = urdf_model.getLink(tip_str);

Replacing
boost::shared_ptr<const urdf::Link> l
with
urdf::LinkConstSharedPtr l

fixes this error.

[rhaschke]

Addressed in #18. Note, that urdfdom 1.0 (that has the transition to std::shared_ptr) is not released yet in any Ubuntu release. Xenial/Kinetic still comes with version 0.4.1.

[nxdefiant]

But Debian testing/unstable already has it. #18 compiles, thanks.