Update Nokogiri Dependency

It would be very helpful for our Jekyll-based Docs site if the `nokogiri` dep for this Gem was updated. Right now html-extractor depends on a fairly old version.

Changing the dep line in the Gemfile from `nokogiri ~-> 1.8.2` to something like `nokogiri >= 1.10` would be great. There's a security vulnerability in Nokogiri that was fixed with the v1.10.4 release (and maybe others from previous releases as well).

Thanks