[bliptv] Filter out SRT files

The bliptv plugins currently fails for videos which have subtitles.
e.g. http://blip.tv/play/h6Uag5OEVgI.html fails with the following traceback:
```
Traceback (most recent call last):
  File "/usr/bin/youtube-dl", line 6, in <module>
    youtube_dl.main()
  File "/usr/lib64/python2.7/site-packages/youtube_dl/__init__.py", line 783, in main
    _real_main(argv)
  File "/usr/lib64/python2.7/site-packages/youtube_dl/__init__.py", line 773, in _real_main
    retcode = ydl.download(all_urls)
  File "/usr/lib64/python2.7/site-packages/youtube_dl/YoutubeDL.py", line 982, in download
    self.extract_info(url)
  File "/usr/lib64/python2.7/site-packages/youtube_dl/YoutubeDL.py", line 510, in extract_info
    return self.process_ie_result(ie_result, download, extra_info)
  File "/usr/lib64/python2.7/site-packages/youtube_dl/YoutubeDL.py", line 546, in process_ie_result
    extra_info=extra_info)
  File "/usr/lib64/python2.7/site-packages/youtube_dl/YoutubeDL.py", line 493, in extract_info
    ie_result = ie.extract(url)
  File "/usr/lib64/python2.7/site-packages/youtube_dl/extractor/common.py", line 158, in extract
    return self._real_extract(url)
  File "/usr/lib64/python2.7/site-packages/youtube_dl/extractor/bliptv.py", line 84, in _real_extract
    if not int(f['media_width']):  # filter m3u8
TypeError: int() argument must be a string or a number, not 'NoneType'
```