Fix logger 'warn' to 'warning' since 'warn' was deprecated in Python 3.3

On Python 3.3, some of the logger tests fail because there's an extra warning being raised:

```
WARNING: DeprecationWarning: The 'warn' method is deprecated, use 'warning' instead [astropy.tests.test_logger]
```

The Python 3.3 docs seem to only talk about ``warning``, not ``warn``, so it must have been deprecated in Python 3.3.

If the tests pass, I'll merge.