Fix for overloaded __ne__ operator

Currently if there are two delorean objects that are identical:

```python
>>> d1 = Delorean(datetime(2013, 1, 1), 'UTC')
>>> d2 = Delorean(datetime(2013, 1, 1), 'UTC')
```

They will evaluate as both equal to and not equal to each other at the same time:

```python
>>> d1 == d2 and d1 != d2
True
>>> # great scott!
```