Bug in TimeSeries.coherence_spectrogram when using nproc>1

There TimeSeries.coherence_spectrogram method errors when using nproc:

>>> from gwpy.timeseries import TimeSeriesDict
>>> data = TimeSeriesDict.fetch(['L1:LSC-SRCL_IN1_DQ', 'L1:LSC-CARM_IN1_DQ'], 'Feb 13 2015', 'Feb 13 2015 00:15')
>>> coh = data['L1:LSC-SRCL_IN1_DQ'].coherence_spectrogram(data['L1:LSC-CARM_IN1_DQ'], 8, 0.5, 0.45, nproc=2)
Process Process-2:
Traceback (most recent call last):
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/multiprocessing/process.py", line 258, in _bootstrap
    self.run()
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/multiprocessing/process.py", line 114, in run
    self._target(*self._args, **self._kwargs)
TypeError: _specgram() takes exactly 2 arguments (3 given)

The mp _specgram method from /gwpy/spectrogram/coherence.py is buggy.