Added support for YAML-CPP 0.5+.

The new yaml-cpp API removes the "node >> outputvar;" operator, and it has a new way of loading documents. There's no version hint in the library's headers, so I'm getting the version number from pkg-config.

See https://github.com/ros-planning/navigation/commit/6c82fb01340ec49b68c47b9c34beb47824782a20 for more info

There was a little bit of "funky" that had to happen in `bwi_mapper/src/nodes/prepare_graph.cpp` around line 33, but other than that, all of the fixes have been implemented in other ROS packages.

Thanks!