Update to use go modules (#671)

This includes updates to CI config, vendor files, etc.
I've hard-coded the k8s depedency at 1.13.4 to keep it inline with what
we currently have and to make sure a another run of `go mod tidy`
doesn't accidentally update it to an unexpected version.

Thanks to hectorj2f for carrying this along.

vendor/sigs.k8s.io/yaml/OWNERS

@@ -0,0 +1,25 @@
+approvers:
+- dims
+- lavalamp
+- smarterclayton
+- deads2k
+- sttts
+- liggitt
+- caesarxuchao
+reviewers:
+- dims
+- thockin
+- lavalamp
+- smarterclayton
+- wojtek-t
+- deads2k
+- derekwaynecarr
+- caesarxuchao
+- mikedanese
+- liggitt
+- gmarek
+- sttts
+- ncdc
+- tallclair
+labels:
+- sig/api-machinery