Fix Icecast hook for ftp:// sites

[...]
  File "/home/archivebot/.local/lib/python3.4/site-packages/wpull/hook.py", line 433, in _handle_pre_response
    url_info_dict, url_record_dict, response_info_dict
  File "wpull_hooks.py", line 208, in handle_pre_response
    if response_info['version'] == 'ICY':
KeyError: 'version'