Sword is compatible only with Ruby 2.0+ and JRuby 9000

Rubinius 2.0+ is supported as well.

JRuby 9000 is not released yet, so sometimes the build can fail. It also
hasn't implemented Module#prepend in MRI-compatible way yet, so until
jruby/jruby#751 is fixed and closed, something related to Module#prepend
could break.