Update data science solution

No need for the `numerator` temporary variable at all here. As long as the first part is a single logical statement (it is), you can leave it alone, as-is:

```
def self.numerator(data_a, data_b)
  (((data_a.conversions * data_b.non_conversions) -
    (data_a.non_conversions * data_b.conversions))**2) *
     (data_a.total_samples + data_b.total_samples)
end