Update golang spdy to new path.

Upstream golang spdy was moved to x/net/spdy and then deleted.  This vendors in
the last revision and updates the only user of it.

Godeps/_workspace/src/github.com/docker/spdystream/connection.go

@@ -9,7 +9,7 @@ import (
 	"sync"
 	"time"
 
-	"code.google.com/p/go.net/spdy"
+	"golang.org/x/net/spdy"
 )
 
 var (

Godeps/_workspace/src/github.com/docker/spdystream/priority.go

@@ -1,9 +1,10 @@
 package spdystream
 
 import (
-	"code.google.com/p/go.net/spdy"
 	"container/heap"
 	"sync"
+
+	"golang.org/x/net/spdy"
 )
 
 type prioritizedFrame struct {

Godeps/_workspace/src/github.com/docker/spdystream/priority_test.go

@@ -1,10 +1,11 @@
 package spdystream
 
 import (
-	"code.google.com/p/go.net/spdy"
 	"sync"
 	"testing"
 	"time"
+
+	"golang.org/x/net/spdy"
 )
 
 func TestPriorityQueueOrdering(t *testing.T) {

Godeps/_workspace/src/github.com/docker/spdystream/stream.go

@@ -9,7 +9,7 @@ import (
 	"sync"
 	"time"
 
-	"code.google.com/p/go.net/spdy"
+	"golang.org/x/net/spdy"
 )
 
 var (