minimal fix for broken dirichlet_mean function

Further, it might be better to write this all in a way that makes it clearer that `dirichlet_mean` is exactly equivalent to calling `bayesian_mean` with the right parameters (except the parameters are hidden in object properties, so it's weird that way too). Here's on hacky way to do it:

```
    def dirichlet_mean(self, arr, prior=PRIOR):
        """
        Computes the Dirichlet mean with a prior.
        """
        self.confidence = float(sum(prior))
        score_sum = sum([val * n for val, n in zip(range(1, 6), prior)])
        self.prior = score_sum / self.confidence
        return self.bayesian_mean(arr)
```