Fix reading in an Amber prmtop file as a topology when it has an ATOMIC_NUMBER section

Fixes the following exception:

```
/.../mdtraj/formats/prmtop.pyc
in load_prmtop(filename)
    200         if 'ATOMIC_NUMBER' in raw_data:
    201             try:
--> 202                 element = elem.Element.get_by_atomic_number(int(raw_data['ATOMIC_NUMBER'][index]))
    203             except KeyError:
    204                 element = None

AttributeError: type object 'Element' has no attribute 'get_by_atomic_number'
```