Fix yaml-cpp to work with new 0.5 API

 - Remove unneeded includes in failed_display.cpp
 - New API which is marginally better/different

 ! Not sure how to detect the difference between 0.3.0 and 0.5.1
 installed on a system.  Debian unstable now has 0.5.1 and rviz
 FTBFS as a result.

I suspect you want to hold off on merging this until Ubuntu also makes the jump to 0.5.x but here it is anyway