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/pickfirst.go

@@ -19,7 +19,8 @@
 package grpc
 
 import (
-	"golang.org/x/net/context"
+	"context"
+
 	"google.golang.org/grpc/balancer"
 	"google.golang.org/grpc/connectivity"
 	"google.golang.org/grpc/grpclog"
@@ -56,6 +57,7 @@ func (b *pickfirstBalancer) HandleResolvedAddrs(addrs []resolver.Address, err er
 	if b.sc == nil {
 		b.sc, err = b.cc.NewSubConn(addrs, balancer.NewSubConnOptions{})
 		if err != nil {
+			//TODO(yuxuanli): why not change the cc state to Idle?
 			grpclog.Errorf("pickfirstBalancer: failed to NewSubConn: %v", err)
 			return
 		}