Missing module in Ai4r::Classifiers::NaiveBayes#build method

```
NameError: uninitialized constant Ai4r::Classifiers::NaiveBayes::DataSet
	from /usr/local/var/rbenv/versions/1.9.3-p392/lib/ruby/gems/1.9.1/gems/ai4r-1.12/lib/ai4r/classifiers/naive_bayes.rb:105:in `build'
```

There is little typo in `Ai4r::Classifiers::NaiveBayes#build` method causing NameError.
PR attached.