outdated MP interface in getting bulk data

Hi developers,
I meet this problem when running `get_MP.py`:
``` python
ImportError: cannot import name 'MPRester' from 'pymatgen' (unknown location)
```

Then I find the interface in  `get_MP.py` is out of date if pymatgen is newest. The new interface is used like:
``` python
from pymatgen.ext.matproj import MPRester
```
And it works.