update build examples in README response code type and value for Jenkins v1.519 differences
changed test http response code to a string to match Net:HTTPResponse
Net::HTTPResponse’s code returns a string, see http://ruby-doc.org/stdlib-2.1.1/libdoc/net/http/rdoc/Net/HTTPResponse.html

I realise this doesn’t change much as the code is currently just passed through, but it could be confusing if the tests are used as examples, or something changes where the code is processed.