don't set number of pool workers

travis build 115 failed twice with the stack trace below:
```
======================================================================
ERROR: test_number_of_events (pypore.sampledata.tests.test_creator.TestCreateRandomData)
Tests that the number of events is roughly correct. The number of events should be random, which is why
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/travis/build/parkin/pypore/src/pypore/tests/util.py", line 28, in wrap
    function(*args, filename=filename, **kwargs)
  File "/home/travis/build/parkin/pypore/src/pypore/sampledata/tests/test_creator.py", line 151, in test_number_of_events
    p = Pool(trials)
  File "/usr/lib/python2.7/multiprocessing/__init__.py", line 232, in Pool
    return Pool(processes, initializer, initargs, maxtasksperchild)
  File "/usr/lib/python2.7/multiprocessing/pool.py", line 136, in __init__
    self._repopulate_pool()
  File "/usr/lib/python2.7/multiprocessing/pool.py", line 199, in _repopulate_pool
    w.start()
  File "/usr/lib/python2.7/multiprocessing/process.py", line 130, in start
    self._popen = Popen(self)
  File "/usr/lib/python2.7/multiprocessing/forking.py", line 120, in __init__
    self.pid = os.fork()
OSError: [Errno 12] Cannot allocate memory
```

Not setting the number of workers in the Pool (to a high number like 10!) should hopefully fix this.