Update vendor dependencies

vendor/golang.org/x/sys/unix/example_test.go

@@ -0,0 +1,19 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build darwin dragonfly freebsd linux netbsd openbsd solaris
+
+package unix_test
+
+import (
+	"log"
+	"os"
+
+	"golang.org/x/sys/unix"
+)
+
+func ExampleExec() {
+	err := unix.Exec("/bin/ls", []string{"ls", "-al"}, os.Environ())
+	log.Fatal(err)
+}