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/encode.go

@@ -37,27 +37,9 @@ package proto
 
 import (
 	"errors"
-	"fmt"
 	"reflect"
 )
 
-// RequiredNotSetError is the error returned if Marshal is called with
-// a protocol buffer struct whose required fields have not
-// all been initialized. It is also the error returned if Unmarshal is
-// called with an encoded protocol buffer that does not include all the
-// required fields.
-//
-// When printed, RequiredNotSetError reports the first unset required field in a
-// message. If the field cannot be precisely determined, it is reported as
-// "{Unknown}".
-type RequiredNotSetError struct {
-	field string
-}
-
-func (e *RequiredNotSetError) Error() string {
-	return fmt.Sprintf("proto: required field %q not set", e.field)
-}
-
 var (
 	// errRepeatedHasNil is the error returned if Marshal is called with
 	// a struct with a repeated field containing a nil element.