Require the correct Yaml version for CMake as well.

The autotools configure script will complain if it can only find a 0.3.0 API version of yaml-cpp when the new 0.5.0 one is required. The CMake project however is happy, but the project then fails to compile.
Merge pull request #781 from JoakimSoderberg/cmake_yaml

Require the correct Yaml version for CMake as well.