Add License to FindGoolgMock.cmake file
Require yaml-cpp version 0.5.1.1

Require users install yaml-cpp version 0.5.1.1, which is our modified
version. This should prevent users from use other packaged version of
yaml-cpp.
Use YAML_CPP_LIBRARIES instead of hard coding the yaml libraries

Previosly, we were hard coding the external yaml-cpp library as a
dependancy. Now we use the variable YAML_CPP_LIBRARIES that is populated
by yaml-cpp's find_package in CMake.