The vinyl-ftp package has a function clean() but I'm not sure how to use it right. I need to:
- get all files from my 
buildfolder - put them into the target folder on my ftp server
 - clean files if they're not available locally
 
I have the following gulp task:
gulp.task('deploy', () => {
  let conn = ftp.create({host:host,user:user,password: password});
  return gulp.src('build/**', {base: './build/', buffer: false })
      .pipe(conn.newer('/path/on/my/server/')) // only upload newer files
      .pipe(conn.dest('/path/on/my/server/'))
      .pipe(conn.clean('build/**', './build/'));
});
1) and 2) is OK, but the clean() function does nothing