Update exception

[DanielOaks]

Got this exception while updating. Related to #14

Exception in thread Thread-5:
Traceback (most recent call last):
  File "<site>/basc_archiver/sites/base.py", line 62, in run
  File "<site>/basc_archiver/sites/fourchan.py", line 219, in download_item
    print('  Thread JSON downloaded.')
  File "<site>/basc_py4chan/thread.py", line 172, in update
    self.replies.extend(Post(self, p) for p in posts if p['no'] > self.last_reply_id)
  File "<site>/basc_py4chan/thread.py", line 172, in <genexpr>
    self.replies.extend(Post(self, p) for p in posts if p['no'] > self.last_reply_id)
KeyError: 'no'