fix installation destination of plugin description

The plugin description used to be installed here:

    /opt/ros/hydro/share/abb_irb2400_moveit_plugins/irb6640_kinematics/abb_irb2400_manipulator_moveit_ikfast_plugin_description.xml

But it's supposed to be here:

    /opt/ros/hydro/share/abb_irb2400_moveit_plugins/irb2400_kinematics/abb_irb2400_manipulator_moveit_ikfast_plugin_description.xml

Also see: https://groups.google.com/d/msg/moveit-users/LQFJwqd0N5I/eThqWkXXRMgJ
Merge pull request #56 from mintar/fix_plugin_desc_install

fix installation destination of plugin description