Fix StatsdHandler._send bug after ce5c6001ae75969f56c8bb5bdc150e6c38104ce2 rewrite

It seems that since [this commit](https://github.com/BrightcoveOS/Diamond/commit/ce5c6001ae75969f56c8bb5bdc150e6c38104ce2#src/diamond/handler/stats_d.py) a bug was introduced on ``StatsdHandler._send`` as it doesn't require the ``metric`` argument anymore.

I found it reading diamond' s logs:

    [2013-04-30 13:40:24,617] [Thread-1] Traceback (most recent call last):
      File "/home/virtualenv/diamond/local/lib/python2.7/site-packages/diamond/handler/Handler.py", line 52, in _flush
        self.flush()
      File "/home/virtualenv/diamond/local/lib/python2.7/site-packages/diamond/handler/stats_d.py", line 98, in flush
        self._send()
    TypeError: _send() takes exactly 2 arguments (1 given)