Add fix for Python Issue 15881 in <2.7.6 <3.2

After adding `django-pq` to my project, my [travis ci tests broke](https://travis-ci.org/saulshanabrook/django-canadanewyork/builds/16978064#L118-L138), because it won't install on Python < 2.7.6 and < 3.2, > 3

To [work around the fix](http://bugs.python.org/issue15881#msg170215) add
```python
try:
    import multiprocessing
except ImportError:
    pass
```
to the top of the `setup.py` file.