vendor: update github.com/pkg/sftp to fix sftp client issues

See: https://forum.rclone.org/t/failed-to-copy-sftp-folder-not-found-c-ftpsites-ssh-fx-failure/9778
See: https://github.com/pkg/sftp/issues/288

vendor/github.com/pkg/sftp/.travis.yml

@@ -4,8 +4,8 @@ go_import_path: github.com/pkg/sftp
 # current and previous stable releases, plus tip
 # remember to exclude previous and tip for macs below
 go:
-  - 1.10.x
   - 1.11.x
+  - 1.12.x
   - tip
 
 os:

vendor/github.com/pkg/sftp/client.go

@@ -884,7 +884,7 @@ func (f *File) Read(b []byte) (int, error) {
 // maximise throughput for transferring the entire file (especially
 // over high latency links).
 func (f *File) WriteTo(w io.Writer) (int64, error) {
-	fi, err := f.Stat()
+	fi, err := f.c.Stat(f.path)
 	if err != nil {
 		return 0, err
 	}