Update yaml-cpp

[ChrisJChang]

Yaml-cpp gives many warnings, and there is a newer release, so we should look at updating to v0.8.0.

[ChrisJChang]

Just a note: This is not trivial, as in v0.8.0 yaml-cpp has removed the capability to do some of the things that we do. e.g. our HasKey(keys...) function won't work when trying to convert yaml nodes to bools.