private method 'load' called for Psych:Module (NoMethodError) にはまった

現象

YAML.loadを利用したアプリケーションを実行すると

/Users/resessh/.rbenv/versions/2.2.2/lib/ruby/gems/2.2.0/gems/ec2ssh-3.0.3/lib/ec2ssh/migrator.rb:17:in 'check_version': private method 'load' called for Psych:Module (NoMethodError)
        from /Users/resessh/.rbenv/versions/2.2.2/lib/ruby/gems/2.2.0/gems/ec2ssh-3.0.3/lib/ec2ssh/cli.rb:96:in 'check_dotfile_version'
        from /Users/resessh/.rbenv/versions/2.2.2/lib/ruby/gems/2.2.0/gems/ec2ssh-3.0.3/lib/ec2ssh/cli.rb:25:in 'update'
        from /Users/resessh/.rbenv/versions/2.2.2/lib/ruby/gems/2.2.0/gems/thor-0.19.1/lib/thor/command.rb:27:in 'run'
        from /Users/resessh/.rbenv/versions/2.2.2/lib/ruby/gems/2.2.0/gems/thor-0.19.1/lib/thor/invocation.rb:126:in 'invoke_command'
        from /Users/resessh/.rbenv/versions/2.2.2/lib/ruby/gems/2.2.0/gems/thor-0.19.1/lib/thor.rb:359:in 'dispatch'
        from /Users/resessh/.rbenv/versions/2.2.2/lib/ruby/gems/2.2.0/gems/thor-0.19.1/lib/thor/base.rb:440:in 'start'
        from /Users/resessh/.rbenv/versions/2.2.2/lib/ruby/gems/2.2.0/gems/ec2ssh-3.0.3/bin/ec2ssh:4:in '<top (required)>'
        from /Users/resessh/.rbenv/versions/2.2.2/bin/ec2ssh:23:in 'load'
        from /Users/resessh/.rbenv/versions/2.2.2/bin/ec2ssh:23:in '<main>'

のように private method 'load' called for Psych:Module (NoMethodError) が吐かれて動作しない。

※ ec2sshいつも便利に使わせてもらってます！

解決策

$ gem update --system

gemのバージョンを2.4.6以上に上げる。

参考文献

Problem installing 3.0.5 on OS X · Issue #584 · ruby-gnome2/ruby-gnome2
Heisen-loading of 2.0.12 · Issue #223 · tenderlove/psych