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/gogo/protobuf/proto/lib_gogo.go

@@ -33,6 +33,14 @@ import (
 	"strconv"
 )
 
+type Sizer interface {
+	Size() int
+}
+
+type ProtoSizer interface {
+	ProtoSize() int
+}
+
 func MarshalJSONEnum(m map[int32]string, value int32) ([]byte, error) {
 	s, ok := m[value]
 	if !ok {