Error downloading font awesome using command example
Running the first [example](https://github.com/Pythonity/icon-font-to-png#examples) I get the error `urllib.error.HTTPError: HTTP Error 404: Not Found`.

On OSX 10.15.2 and Python 3.5.6:

(env) nathan@Nathans-MBP fontawesometopng % icon-font-to-png --download font-awesome           
Traceback (most recent call last):

  File "/Users/nathan/Playground/fontawesometopng/env/bin/icon-font-to-png", line 16, in <module>
    command_line.run(sys.argv[1:])
  File "/Users/nathan/Playground/fontawesometopng/env/lib/python3.5/site-packages/icon_font_to_png/command_line.py", line 85, in run
    downloader = download_icon_font(args.download, os.getcwd())
  File "/Users/nathan/Playground/fontawesometopng/env/lib/python3.5/site-packages/icon_font_to_png/command_line.py", line 172, in download_icon_font
    downloader.download_files()
  File "/Users/nathan/Playground/fontawesometopng/env/lib/python3.5/site-packages/icon_font_to_png/icon_font_downloader.py", line 75, in download_files
    self.css_path = self.download_css(self.directory)
  File "/Users/nathan/Playground/fontawesometopng/env/lib/python3.5/site-packages/icon_font_to_png/icon_font_downloader.py", line 67, in download_css
    return self._download_file_from_url(self.css_url, directory)
  File "/Users/nathan/Playground/fontawesometopng/env/lib/python3.5/site-packages/icon_font_to_png/icon_font_downloader.py", line 49, in _download_file_from_url
    return urlretrieve(url, filename=css_filename)[0]
  File "/Users/nathan/.pyenv/versions/3.5.6/lib/python3.5/urllib/request.py", line 188, in urlretrieve
    with contextlib.closing(urlopen(url, data)) as fp:
  File "/Users/nathan/.pyenv/versions/3.5.6/lib/python3.5/urllib/request.py", line 163, in urlopen
    return opener.open(url, data, timeout)
  File "/Users/nathan/.pyenv/versions/3.5.6/lib/python3.5/urllib/request.py", line 472, in open
    response = meth(req, response)
  File "/Users/nathan/.pyenv/versions/3.5.6/lib/python3.5/urllib/request.py", line 582, in http_response
    'http', request, response, code, msg, hdrs)
  File "/Users/nathan/.pyenv/versions/3.5.6/lib/python3.5/urllib/request.py", line 510, in error
    return self._call_chain(*args)
  File "/Users/nathan/.pyenv/versions/3.5.6/lib/python3.5/urllib/request.py", line 444, in _call_chain
    result = func(*args)
  File "/Users/nathan/.pyenv/versions/3.5.6/lib/python3.5/urllib/request.py", line 590, in http_error_default
    raise HTTPError(req.full_url, code, msg, hdrs, fp)
urllib.error.HTTPError: HTTP Error 404: Not Found