As of ROS Electric, the Yamlcpp package has been replaced with a system dependency. Old documentation
From version 0.5, yaml-cpp started using boost as a dependency. If you'd like to continue using the non-boost variant (useful on embedded boards), pull in the yaml-cpp-0-3 ros third party dependency. Note that ubuntu does provide a 0.3 variant, but it cannot be used in parallel. The ros third party dependency namespaces so that you can - see the README.
Starting in ROS Electric, Yamlcpp is installed as a rosdep system dependency. This means that you compile against Yamlcpp without requiring any ROS-specific build tools.
ROS Electric: using the system install
You need to explicitly link against against the yaml-cpp library:
If you have previously used yaml_cpp with ROS, make sure you remove <depend package="yaml_cpp" /> from your manifest.
<depend package="common_rosdeps" /> <rosdep name="yaml-cpp" />
<depend stack="common_rosdeps" />
Please see the yaml-cpp homepage for documentation.