Fix resource collection previous index

After upgrading to ReviewBoard 2.0, we were getting errors from tools that query user lists.  Making this change fixed it.
Old error (btw, happens the same on our prototype server which uses python 2.7):
Traceback (most recent call last):

...
File "/usr/lib/python2.6/site-packages/Djblets-0.8.4-py2.6.egg/djblets/webapi/responses.py", line 211, in get_prev_index

    return max(self.start - self.max_results)
TypeError: 'int' object is not iterable