Detect yaml-cpp >= 0.5 and use the new API.

The current release of yaml-cpp, 0.5[.1], has an incompatible API.
There is no version information in the headers, so I'm using the
version from pkg-config to figure out which API is appropriate.
Merge pull request #16 from ktossell/hydro-devel

Detect yaml-cpp >= 0.5 and use the new API.