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/oauth2/token.go

@@ -5,6 +5,7 @@
 package oauth2
 
 import (
+	"context"
 	"fmt"
 	"net/http"
 	"net/url"
@@ -12,7 +13,6 @@ import (
 	"strings"
 	"time"
 
-	"golang.org/x/net/context"
 	"golang.org/x/oauth2/internal"
 )
 
@@ -118,13 +118,16 @@ func (t *Token) Extra(key string) interface{} {
 	return v
 }
 
+// timeNow is time.Now but pulled out as a variable for tests.
+var timeNow = time.Now
+
 // expired reports whether the token is expired.
 // t must be non-nil.
 func (t *Token) expired() bool {
 	if t.Expiry.IsZero() {
 		return false
 	}
-	return t.Expiry.Round(0).Add(-expiryDelta).Before(time.Now())
+	return t.Expiry.Round(0).Add(-expiryDelta).Before(timeNow())
 }
 
 // Valid reports whether t is non-nil, has an AccessToken, and is not expired.
@@ -151,7 +154,7 @@ func tokenFromInternal(t *internal.Token) *Token {
 // This token is then mapped from *internal.Token into an *oauth2.Token which is returned along
 // with an error..
 func retrieveToken(ctx context.Context, c *Config, v url.Values) (*Token, error) {
-	tk, err := internal.RetrieveToken(ctx, c.ClientID, c.ClientSecret, c.Endpoint.TokenURL, v)
+	tk, err := internal.RetrieveToken(ctx, c.ClientID, c.ClientSecret, c.Endpoint.TokenURL, v, internal.AuthStyle(c.Endpoint.AuthStyle))
 	if err != nil {
 		if rErr, ok := err.(*internal.RetrieveError); ok {
 			return nil, (*RetrieveError)(rErr)