Can't run the detailed example in plain Ruby project

[olimart]

As always, very good stuff Andrew 👏

When running the detailed project in plain Ruby file, it breaks at the save section because of dependency loading/resolution.
uninitialized constant Eps::LinearRegression::Nokogiri

I noticed Nokogiri is declared only for dev. environment and don't know if you plan to keep as-is or maybe declare as top-level ::Nokogiri

[ankane]

Thanks @olimart, I've added require "nokogiri" to the to_pmml method. I've considered making Nokogiri a runtime dependency of the gem, but it's only need for persisting models, so decided against it for now.