FIX disable memmapping for object arrays

Fix for #162. Otherwise this can cause errors such as:

```
...
  File "/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/pickle.py", line 1036, in load
    dispatch[key[0]](self)
  File "/Users/ogrisel/code/joblib/joblib/numpy_pickle.py", line 291, in load_build
    array = nd_array_wrapper.read(self)
  File "/Users/ogrisel/code/joblib/joblib/numpy_pickle.py", line 113, in read
    mmap_mode=unpickler.mmap_mode)
  File "/Users/ogrisel/code/numpy/numpy/lib/npyio.py", line 389, in load
    return format.open_memmap(file, mode=mmap_mode)
  File "/Users/ogrisel/code/numpy/numpy/lib/format.py", line 684, in open_memmap
    raise ValueError(msg)
ValueError: Array can't be memory-mapped: Python objects in dtype.
```
Good idea.