Switch from x/net/context -> context

Since Go 1.7, context is a standard package. Since Go 1.9, everything
that is provided by "x/net/context" is a couple of type aliases to
types in "context".

Many vendored packages still use x/net/context, so vendor entry remains
for now.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

integration-cli/docker_api_containers_test.go

@@ -3,6 +3,7 @@ package main
 import (
 	"archive/tar"
 	"bytes"
+	"context"
 	"encoding/json"
 	"fmt"
 	"io"
@@ -34,7 +35,6 @@ import (
 	"github.com/gotestyourself/gotestyourself/assert"
 	is "github.com/gotestyourself/gotestyourself/assert/cmp"
 	"github.com/gotestyourself/gotestyourself/poll"
-	"golang.org/x/net/context"
 )
 
 func (s *DockerSuite) TestContainerAPIGetAll(c *check.C) {