Move to gopkg.in for YAML

The launchpad repository for YAML is deprecated and has moved to GitHub (go-yaml/yaml). The maintainer, @niemeyer, is also the maintainer of gopkg.in and recommends that you use it for the YAML package.

Another advantage, albeit mostly theoretical: if there were ever a v2 of the YAML package, gopkg.in lets you use `go get github.com/mediocregopher/goat` without risking breakage.

The APIs are the same for these packages, so there's no real code changes.