Update vendor dir

vendor/github.com/xenolf/lego/log/logger.go

@@ -6,54 +6,54 @@ import (
 )
 
 // Logger is an optional custom logger.
-var Logger *log.Logger
+var Logger StdLogger = log.New(os.Stdout, "", log.LstdFlags)
+
+// StdLogger interface for Standard Logger.
+type StdLogger interface {
+	Fatal(args ...interface{})
+	Fatalln(args ...interface{})
+	Fatalf(format string, args ...interface{})
+	Print(args ...interface{})
+	Println(args ...interface{})
+	Printf(format string, args ...interface{})
+}
 
 // Fatal writes a log entry.
 // It uses Logger if not nil, otherwise it uses the default log.Logger.
 func Fatal(args ...interface{}) {
-	if Logger == nil {
-		Logger = log.New(os.Stderr, "", log.LstdFlags)
-	}
-
 	Logger.Fatal(args...)
 }
 
 // Fatalf writes a log entry.
 // It uses Logger if not nil, otherwise it uses the default log.Logger.
 func Fatalf(format string, args ...interface{}) {
-	if Logger == nil {
-		Logger = log.New(os.Stderr, "", log.LstdFlags)
-	}
-
 	Logger.Fatalf(format, args...)
 }
 
 // Print writes a log entry.
 // It uses Logger if not nil, otherwise it uses the default log.Logger.
 func Print(args ...interface{}) {
-	if Logger == nil {
-		Logger = log.New(os.Stdout, "", log.LstdFlags)
-	}
-
 	Logger.Print(args...)
 }
 
 // Println writes a log entry.
 // It uses Logger if not nil, otherwise it uses the default log.Logger.
 func Println(args ...interface{}) {
-	if Logger == nil {
-		Logger = log.New(os.Stdout, "", log.LstdFlags)
-	}
-
 	Logger.Println(args...)
 }
 
 // Printf writes a log entry.
 // It uses Logger if not nil, otherwise it uses the default log.Logger.
 func Printf(format string, args ...interface{}) {
-	if Logger == nil {
-		Logger = log.New(os.Stdout, "", log.LstdFlags)
-	}
-
 	Logger.Printf(format, args...)
 }
+
+// Warnf writes a log entry.
+func Warnf(format string, args ...interface{}) {
+	Printf("[WARN] "+format, args...)
+}
+
+// Infof writes a log entry.
+func Infof(format string, args ...interface{}) {
+	Printf("[INFO] "+format, args...)
+}