longrunning: use 1.6 context

Use golang.org/x/net/context for Go 1.6 compatibility.

Change-Id: If1e22369498914a871bf247b42dc6e65c06c1161
Reviewed-on: https://code-review.googlesource.com/8570
Reviewed-by: Michael Darakananda <pongad@google.com>

longrunning/example_test.go

@@ -15,13 +15,13 @@
 package longrunning
 
 import (
-	"context"
 	"fmt"
 	"time"
 
 	"github.com/golang/protobuf/ptypes"
 	"github.com/golang/protobuf/ptypes/duration"
 	"github.com/golang/protobuf/ptypes/timestamp"
+	"golang.org/x/net/context"
 	pb "google.golang.org/genproto/googleapis/longrunning"
 )