[8tracks] Use track count instead of looking at at_last_track property

This fixes the error:

$ youtube-dl http://8tracks.com/vladmc/counting-stars
[8tracks] counting-stars: Downloading webpage
[8tracks] counting-stars: Downloading song information 1/4
[8tracks] counting-stars: Downloading song information 2/4
[8tracks] counting-stars: Downloading song information 3/4
[8tracks] counting-stars: Downloading song information 4/4
[8tracks] counting-stars: Downloading song information 5/4
Traceback (most recent call last):
  File "/usr/lib/python2.7/runpy.py", line 162, in _run_module_as_main
    "__main__", fname, loader, pkg_name)
  File "/usr/lib/python2.7/runpy.py", line 72, in _run_code
    exec code in run_globals
  File "/home/phihag/projects/youtube-dl/youtube_dl/__main__.py", line 18, in <module>
    youtube_dl.main()
  File "/home/phihag/projects/youtube-dl/youtube_dl/__init__.py", line 761, in main
    _real_main(argv)
  File "/home/phihag/projects/youtube-dl/youtube_dl/__init__.py", line 714, in _real_main
    retcode = ydl.download(all_urls)
  File "/home/phihag/projects/youtube-dl/youtube_dl/YoutubeDL.py", line 701, in download
    videos = self.extract_info(url)
  File "/home/phihag/projects/youtube-dl/youtube_dl/YoutubeDL.py", line 342, in extract_info
    ie_result = ie.extract(url)
  File "/home/phihag/projects/youtube-dl/youtube_dl/extractor/common.py", line 121, in extract
    return self._real_extract(url)
  File "/home/phihag/projects/youtube-dl/youtube_dl/extractor/eighttracks.py", line 111, in _real_extract
    'id': track_data['id'],
KeyError: 'id'