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/github.com/golang/protobuf/proto/pointer_reflect.go

@@ -79,10 +79,13 @@ func toPointer(i *Message) pointer {
 
 // toAddrPointer converts an interface to a pointer that points to
 // the interface data.
-func toAddrPointer(i *interface{}, isptr bool) pointer {
+func toAddrPointer(i *interface{}, isptr, deref bool) pointer {
 	v := reflect.ValueOf(*i)
 	u := reflect.New(v.Type())
 	u.Elem().Set(v)
+	if deref {
+		u = u.Elem()
+	}
 	return pointer{v: u}
 }