Fix limit=None bug in replace_more_comments

This fixes the following error, which came from trying to decrement `remaining` when it was `None`.

```python
>>> import praw
>>> r = praw.Reddit('bug example for limit=None by u/_Daimon_')
>>> s = r.get_submission('http://www.reddit.com/r/AskReddit/comments/16r8tc/what_
little_tricks_do_you_play_on_your_so/')
>>> s.replace_more_comments(limit=None, threshold=20)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "c:\Python27\lib\site-packages\praw\objects.py", line 849, in replace_more
_comments
    remaining -= 1
TypeError: unsupported operand type(s) for -=: 'NoneType' and 'int'
>>>
```