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/golang.org/x/sync/semaphore/semaphore.go

@@ -7,12 +7,8 @@ package semaphore // import "golang.org/x/sync/semaphore"
 
 import (
 	"container/list"
+	"context"
 	"sync"
-
-	// Use the old context because packages that depend on this one
-	// (e.g. cloud.google.com/go/...) must run on Go 1.6.
-	// TODO(jba): update to "context" when possible.
-	"golang.org/x/net/context"
 )
 
 type waiter struct {
@@ -36,9 +32,9 @@ type Weighted struct {
 	waiters list.List
 }
 
-// Acquire acquires the semaphore with a weight of n, blocking only until ctx
-// is done. On success, returns nil. On failure, returns ctx.Err() and leaves
-// the semaphore unchanged.
+// Acquire acquires the semaphore with a weight of n, blocking until resources
+// are available or ctx is done. On success, returns nil. On failure, returns
+// ctx.Err() and leaves the semaphore unchanged.
 //
 // If ctx is already done, Acquire may still succeed without blocking.
 func (s *Weighted) Acquire(ctx context.Context, n int64) error {