Don't throw an exception when hostname is None

[mwilson-ns]

$ lsi -c 'ps aux | grep harvest' 
...

Enter command (h for help, q to quit): x

Running command `ps aux | grep harvest` on 415 matching hosts
Traceback (most recent call last):
  File "/home/mwilson/.pyenv/versions/2.7.3/bin/lsi", line 9, in <module>
    load_entry_point('lsi==0.1.2', 'console_scripts', 'lsi')()
  File "/home/mwilson/.pyenv/versions/2.7.3/lib/python2.7/site-packages/lsi/lsi.py", line 607, in main
    args.only, args.sort_by, args.limit)
  File "/home/mwilson/.pyenv/versions/2.7.3/lib/python2.7/site-packages/lsi/lsi.py", line 253, in _run_ssh
    return _run_ssh_command(entries, username, idfile, command)
  File "/home/mwilson/.pyenv/versions/2.7.3/lib/python2.7/site-packages/lsi/lsi.py", line 487, in _run_ssh_command
    cmd = _build_ssh_command(hname, username, idfile, command)
  File "/home/mwilson/.pyenv/versions/2.7.3/lib/python2.7/site-packages/lsi/lsi.py", line 334, in _build_ssh_command