gherkin 2.4.8 install error on jruby 1.6.1 and 1.6.3

error.log

$ gem install gherkin --version 2.4.8 --verbose --debug --backtrace
Exception `LoadError' at org/jruby/RubyKernel.java:1038 - no such file to load -- psych
Exception `Gem::LoadError' at /Users/ahellesoy/.rvm/rubies/jruby-1.6.3/lib/ruby/site_ruby/1.8/rubygems.rb:861 - RubyGem version error: sources(0.0.1 not > 0.0.1)

GET http://rubygems.org/specs.4.8.gz
302 Found
GET http://production.s3.rubygems.org/specs.4.8.gz
304 Not Modified
ERROR:  While executing gem ... (ArgumentError)
    undefined class/module YAML::Syck::DefaultKey
	org/jruby/RubyMarshal.java:148:in `load'
	/Users/ahellesoy/.rvm/rubies/jruby-1.6.3/lib/ruby/site_ruby/1.8/rubygems/specification.rb:289:in `_load'
	org/jruby/RubyMarshal.java:148:in `load'
	/Users/ahellesoy/.rvm/rubies/jruby-1.6.3/lib/ruby/site_ruby/1.8/rubygems/spec_fetcher.rb:130:in `fetch_spec'
	/Users/ahellesoy/.rvm/rubies/jruby-1.6.3/lib/ruby/site_ruby/1.8/rubygems/spec_fetcher.rb:86:in `fetch_with_errors'
	org/jruby/RubyArray.java:2336:in `collect'
	/Users/ahellesoy/.rvm/rubies/jruby-1.6.3/lib/ruby/site_ruby/1.8/rubygems/spec_fetcher.rb:85:in `fetch_with_errors'
	/Users/ahellesoy/.rvm/rubies/jruby-1.6.3/lib/ruby/site_ruby/1.8/rubygems/dependency_installer.rb:108:in `find_gems_with_sources'
	/Users/ahellesoy/.rvm/rubies/jruby-1.6.3/lib/ruby/site_ruby/1.8/rubygems/dependency_installer.rb:212:in `find_spec_by_name_and_version'
	/Users/ahellesoy/.rvm/rubies/jruby-1.6.3/lib/ruby/site_ruby/1.8/rubygems/dependency_installer.rb:244:in `install'
	/Users/ahellesoy/.rvm/rubies/jruby-1.6.3/lib/ruby/site_ruby/1.8/rubygems/commands/install_command.rb:120:in `execute'
	org/jruby/RubyArray.java:1603:in `each'
	/Users/ahellesoy/.rvm/rubies/jruby-1.6.3/lib/ruby/site_ruby/1.8/rubygems/commands/install_command.rb:115:in `execute'
	/Users/ahellesoy/.rvm/rubies/jruby-1.6.3/lib/ruby/site_ruby/1.8/rubygems/command.rb:278:in `invoke'
	/Users/ahellesoy/.rvm/rubies/jruby-1.6.3/lib/ruby/site_ruby/1.8/rubygems/command_manager.rb:133:in `process_args'
	/Users/ahellesoy/.rvm/rubies/jruby-1.6.3/lib/ruby/site_ruby/1.8/rubygems/command_manager.rb:103:in `run'
	/Users/ahellesoy/.rvm/rubies/jruby-1.6.3/lib/ruby/site_ruby/1.8/rubygems/gem_runner.rb:63:in `run'
	/Users/ahellesoy/.rvm/rubies/jruby-1.6.3/bin/gem:25:in `(root)'
Exception `Gem::SystemExitException' at /Users/ahellesoy/.rvm/rubies/jruby-1.6.3/lib/ruby/site_ruby/1.8/rubygems/user_interaction.rb:328 - Exiting RubyGems with exit_code 1