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/google.golang.org/grpc/balancer.go

@@ -19,11 +19,10 @@
 package grpc
 
 import (
-	"fmt"
+	"context"
 	"net"
 	"sync"
 
-	"golang.org/x/net/context"
 	"google.golang.org/grpc/codes"
 	"google.golang.org/grpc/credentials"
 	"google.golang.org/grpc/grpclog"
@@ -118,26 +117,6 @@ type Balancer interface {
 	Close() error
 }
 
-// downErr implements net.Error. It is constructed by gRPC internals and passed to the down
-// call of Balancer.
-type downErr struct {
-	timeout   bool
-	temporary bool
-	desc      string
-}
-
-func (e downErr) Error() string   { return e.desc }
-func (e downErr) Timeout() bool   { return e.timeout }
-func (e downErr) Temporary() bool { return e.temporary }
-
-func downErrorf(timeout, temporary bool, format string, a ...interface{}) downErr {
-	return downErr{
-		timeout:   timeout,
-		temporary: temporary,
-		desc:      fmt.Sprintf(format, a...),
-	}
-}
-
 // RoundRobin returns a Balancer that selects addresses round-robin. It uses r to watch
 // the name resolution updates and updates the addresses available correspondingly.
 //
@@ -410,7 +389,3 @@ func (rr *roundRobin) Close() error {
 type pickFirst struct {
 	*roundRobin
 }
-
-func pickFirstBalancerV1(r naming.Resolver) Balancer {
-	return &pickFirst{&roundRobin{r: r}}
-}