yaml-cpp: improving dependencies (resolves #328)

I noticed a build failure for yaml-cpp (#328), that the boost
headers could not be found. Probably since the update to 0.5.2 [1],
yaml-cpp depends on boost. This commit resolves the issue by adding
this dependency.

[1] https://github.com/bmwcarit/meta-ros/commit/da10180fc561ba5da0271476f013d73b6733ea51



Signed-off-by: Lukas Bulwahn <lukas.bulwahn@oss.bmw-carit.de>