Update gem dependencies and versions
Nokogiri is added since it is called directly
ActiveSupport version is relaxed since called methods were introduced in 1.0.0
Builder and Nokogiri versions are relaxed to the minimum of what Savon requires
Use blank wording rather than nil/empty
Update development gems
Wrap XML content in a Nokogiri document
Nokogiri doesn't have the ability to schema validate against an element, so the adaptor will provide a document