Silence pending deprication warning

Currently, `compute_contacts` with numpy 1.8.1 warns:
```
/home/rmcgibbo/miniconda2/lib/python2.7/site-packages/mdtraj-0.8.1-py2.7-linux-x86_64.egg/mdtraj/geometry/contact.py:177: DeprecationWarning: using a non-integer number instead of an integer will result in an error in the future
  distances[:, i] = atom_distances[:, index : index + n].min(axis=1)
```

I'm not really sure what the purpose of this is, since `index` is a numpy.int64. But turning it into a python int seems to do the trick to remove the warning.