XMLRPC is now removed from stdlib

[chulkilee]

• https://www.ruby-lang.org/en/news/2016/12/25/ruby-2-4-0-released/
• https://bugs.ruby-lang.org/issues/12160

To use this gem with Ruby 2.4, include gem 'xmlrpc'. I think it's better to specify xmlrpc dependency in softlayer_api gem, but I'm not sure it would work with older ruby versions since at this moment xmlrpc require ruby versions >= 2.3. Maybe just add it to README.md