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/k8s.io/client-go/pkg/apis/clientauthentication/OWNERS

@@ -0,0 +1,7 @@
+# approval on api packages bubbles to api-approvers
+reviewers:
+- sig-auth-authenticators-approvers
+- sig-auth-authenticators-reviewers
+labels:
+- sig/auth
+