You are probably using Ruby 1.8.7 with a too recent versions of Rubygems.
Downgrade your Rubygems to the latest version that works with 1.8.7.
See also
Posted by Martin Straub to makandra dev (2014-05-14 07:24)
You are probably using Ruby 1.8.7 with a too recent versions of Rubygems.
Downgrade your Rubygems to the latest version that works with 1.8.7.