fix a logging problem in MetadataCommunity

Fixes this backtrace:
```
ERROR   1418250960.63       community:2188  exception during check_callback for metadata
Traceback (most recent call last):
  File "/home/jenkins/workspace/GH_Tribler_pull-request-tester_devel/tribler/Tribler/dispersy/community.py", line 2186, in on_messages
    possibly_messages = list(meta.check_callback(messages))
  File "/home/jenkins/workspace/GH_Tribler_pull-request-tester_devel/tribler/Tribler/community/metadata/community.py", line 144, in check_metadata
    message.candidate.sock_addr[0], message.candidate.sock_addr[1])
  File "/usr/lib/python2.7/logging/__init__.py", line 1148, in debug
    self._log(DEBUG, msg, args, **kwargs)
  File "/usr/lib/python2.7/logging/__init__.py", line 1279, in _log
    self.handle(record)
  File "/usr/lib/python2.7/logging/__init__.py", line 1289, in handle
    self.callHandlers(record)
  File "/usr/lib/python2.7/logging/__init__.py", line 1329, in callHandlers
    hdlr.handle(record)
  File "/usr/lib/python2.7/logging/__init__.py", line 757, in handle
    self.emit(record)
  File "/usr/lib/python2.7/dist-packages/nose/plugins/logcapture.py", line 82, in emit
    self.buffer.append(self.format(record))
  File "/usr/lib/python2.7/logging/__init__.py", line 732, in format
    return fmt.format(record)
  File "/usr/lib/python2.7/logging/__init__.py", line 471, in format
    record.message = record.getMessage()
  File "/usr/lib/python2.7/logging/__init__.py", line 335, in getMessage
    msg = msg % self.args
TypeError: not all arguments converted during string formatting
```