AttributeError: 'Riak' object has no attribute 'timeout_event'

This PR fixes the following error:

    2013-06-17 12:44:39,239 | ERROR | dd.collector | checks.riak(__init__.py:454) | Check 'riak' instance #0 failed
    Traceback (most recent call last):
      File "/usr/share/datadog/agent/checks/__init__.py", line 445, in run
        self.check(instance)
      File "/usr/share/datadog/agent/checks.d/riak.py", line 68, in check
        self.timeout_event(url, timeout, aggregation_key)
    AttributeError: 'Riak' object has no attribute 'timeout_event'

Note: I don't code python so not sure about validity of this fix.