Fix 3.0 - 3.2 incompatibility in save_module

From #59. I think this ought to fix the problem.

```pycon
dude@hilbert>$ python31
dude@hilbert>$ python test_file.py 
Traceback (most recent call last):
  File "/Users/mmckerns/dev/svn/pathos/dill/dill/branch/dill.py", line 886, in save_module
    prefix = sys.base_prefix if PY3 else sys.prefix
AttributeError: 'module' object has no attribute 'base_prefix'
```