Fixed missing return statement in clean_cache_backend method which resulted in empty cache backend and an error accessing /admin page 

settings.CACHE_BACKEND was None resulting in:


  File ".../reviewboard/admin/views.py", line 33, in dashboard
    'has_cache_stats': get_has_cache_stats(),
  File ".../reviewboard/admin/cache_stats.py", line 37, in get_has_cache_stats
    return get_memcached_hosts() != None
  File ".../reviewboard/admin/cache_stats.py", line 25, in get_memcached_hosts
    scheme, host, params = parse_backend_uri(settings.CACHE_BACKEND)
  File ".../django/core/cache/__init__.py", line 59, in parse_backend_uri
    if backend_uri.find(':') == -1:
AttributeError: 'NoneType' object has no attribute 'find'

Changing cache backend in admin interface works as expected.