Adapt to Astropy rename NonLinearLSQFitter to LevMarLSQFitter

At the moment all tests in `photutils/detection/tests/test_morphology.py `, `photutils/tests/test_psf_photometry.py` and `photutils/tests/test_psfs.py` fail like this:
```
>       self.fitter = fitting.NonLinearLSQFitter()
E       AttributeError: 'module' object has no attribute 'NonLinearLSQFitter'
```
https://travis-ci.org/astropy/photutils/jobs/27266795#L603

The reason is that `NonLinearLSQFitter` was renamed to [LevMarLSQFitter](http://astropy.readthedocs.org/en/latest/api/astropy.modeling.fitting.LevMarLSQFitter.html)  in https://github.com/astropy/astropy/pull/2441 .

Once Astropy 0.4 is released we decided to only support the new version, but if the call signature didn't change we could just support both.