Deprecation warning for Daru::Vector#n_valid 

Hi, I played around with https://github.com/appfolio/abprof and got lots of deprecation warnings for statsample:

```
NOTE: Daru::Vector#n_valid is deprecated; use count_values instead. It will be removed on or after 2016-10-01.
Daru::Vector#n_valid called from .rvm/gems/ruby-2.3.1/gems/statsample-2.0.2/lib/statsample/test/t.rb:267
```
