Convert matchers to Nokogiri

This is pull request is my work on https://github.com/wvanbergen/adyen/issues/60. I just converted initializing the REXML::Document to a Nokogiri::XML.parse call and changed the check method of the RSpec matcher to return if the xpath matcher returns any nodes.

Is there a reason why REXML was used and why Nokogiri shouldn't be needed as an external dependency?

Also the matchers do not have any tests. Is that ok?