Protect call to mp.Queue

Error found while running on travis:

```
======================================================================
ERROR: None (tables.tests.test_basics.BloscSubprocess)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/travis/build/joshmoore/PyTables/tables/tests/common.py", line , in newmethod
    return oldmethod(self, *args, **kwargs)
  File "/home/travis/build/joshmoore/PyTables/tables/tests/test_basics.py",
line 2491, in test_multiprocess
    qout = mp.Queue()
  File "/usr/lib/python2.6/multiprocessing/__init__.py", line 213, in Queue
    return Queue(maxsize)
  File "/usr/lib/python2.6/multiprocessing/queues.py", line 37, in __init__
    self._rlock = Lock()
  File "/usr/lib/python2.6/multiprocessing/synchronize.py", line 117, in __init__
    SemLock.__init__(self, SEMAPHORE, 1, 1)
  File "/usr/lib/python2.6/multiprocessing/synchronize.py", line 49, in __init__
    sl = self._semlock = _multiprocessing.SemLock(kind, value, maxvalue)
OSError: [Errno 13] Permission denied

----------------------------------------------------------------------
Ran 5228 tests in 369.499s

FAILED (errors=1)
```

See:
 * https://travis-ci.org/joshmoore/PyTables/jobs/4824033
 * http://stackoverflow.com/questions/2009278/python-multiprocessing-permission-denied