Change comparison to None to get rid of FutureWarning

The attached commit gets rid of this `FutureWarning` I get all the time from `mystic` with `numpy 1.9`:
```
/Users/deil/Library/Python/2.7/lib/python/site-packages/mystic-0.2a2.dev-py2.7.egg/mystic/abstract_solver.py:380: FutureWarning: comparison to `None` will result in an elementwise object comparison in the future.
  if min == None: min = self._defaultMin
```

By the way ... what's the correct way to run the `mystic` tests?
Simply running `nosetests` I get [a ton of errors](https://gist.github.com/cdeil/0ded97d88acd210fed93), so probably that's not the correct invocation?