fix renaming a list of HDFS files to a single dest

I just ran into a problem renaming a list of files to a single destination folder:

```
Runtime error:
Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/luigi/worker.py", line 240, in _run_task
    task.run()
  File "./parse-nginx.py", line 170, in run
    hdfs.rename(paths, dest)
  File "/usr/lib/python2.7/site-packages/luigi/hdfs.py", line 111, in rename
    call_check([load_hadoop_cmd(), 'fs', '-mv', path, dest])
  File "/usr/lib/python2.7/site-packages/luigi/hdfs.py", line 44, in call_check
    p = subprocess.Popen(command, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
  File "/usr/lib64/python2.7/subprocess.py", line 711, in __init__
    errread, errwrite)
  File "/usr/lib64/python2.7/subprocess.py", line 1308, in _execute_child
    raise child_exception
TypeError: execv() arg 2 must contain only strings
```

This patch fixed it for me.