django_histogram.utils.Histogram assert statement is invalid

```
assert months or days, 'You must pass either months or days, not both.'
```

The above line check to see that months or days is in the list of args. It doesn't actually check that only one is used. This would work instead:

```
assert months or days, "You must have months or days"
assert not (months and days), "You must pass either months or days, not both"
```