Fix mol2 warnings

```
rmcgibbo@dn0a230977 ~/projects/mdtraj
$ PYTHONWARNINGS=error nosetests MDTraj/tests/test_mol2.py
...S..
----------------------------------------------------------------------
Ran 6 tests in 0.077s

OK (SKIP=1)
```
specifying engine='python' here and below gets rid of the warning that the default 'c' engine doesn't support regex seperators.