log: remove crusty context, replace with shiny

Signed-off-by: Stephen J Day <stephen.day@docker.com>

log/context.go

@@ -1,10 +1,10 @@
 package log
 
 import (
+	"context"
 	"path"
 
 	"github.com/Sirupsen/logrus"
-	"golang.org/x/net/context"
 )
 
 var (

log/context_test.go

@@ -1,10 +1,10 @@
 package log
 
 import (
+	"context"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
-	"golang.org/x/net/context"
 )
 
 func TestLoggerContext(t *testing.T) {

log/grpc.go

@@ -1,7 +1,8 @@
 package log
 
 import (
-	"golang.org/x/net/context"
+	"context"
+
 	"google.golang.org/grpc/grpclog"
 )