s/process/commit/ in batch._put.

Fixes the following backtrace:

  File "../python2.7/site-packages/starbase/client/table/batch.py", line 59, in insert
    return self._put(row, columns, timestamp=timestamp, encode_content=True)
  File "../python2.7/site-packages/starbase/client/table/batch.py", line 56, in _put
    self.process()
AttributeError: 'Batch' object has no attribute 'process'