Do not install this gem. It contains a malicious code.
It was made to be a demonstration at this blog post: http://netengine.com.au/blog/ruby-conf-australia-2013-in-review-hacking-with-gems/
It will post your whoami command to http://cryptic-ocean-8847.herokuapp.com/usernames.
You can check line 20 from xmastree.gemspec asking to run Rakefile once this gem is installed and Rakefile, from line 11 to 21, will post to the URL above said.