Replace HTML::Document with Nokogiri

Inject into Rails::Dom::Testing::Assertions::DomAssertions

Explicit require statement increases the likelihood that further changes in
module names will be detected early.

Gem dependency on rails-dom-testing will ensure that this change is only
picked up with versions of Rails that are compatible with rails-dom-testing.
Merge pull request #169 from rubys/42-selector-assertions-take2

Replace HTML::Document with Nokogiri