Remove `github.com/ghodss/yaml` dependency

[Juneezee]

At the time of making this commit, the package github.com/ghodss/yaml is no longer actively maintained.

We are only using github.com/ghodss/yaml in common/auth/mapping-rule.go. We can use gopkg.in/yaml.v2 instead.

[cdujeu]

nice spot although I think the dependency is in fact only used by the corresponding test file (not the actual code)