I try to run jekyll and get this warning “Jekyll - uninitialized constant”, i search and found the solution on github

I follow these steps:

~$ jekyll serve

    /home/my-user/.gem/ruby/2.2.0/gems/safe_yaml-1.0.4/lib/safe_yaml/psych_resolver.rb:4:in `<class:PsychResolver>': uninitialized constant Psych::Nodes (NameError)
    from /home/my-user/.gem/ruby/2.2.0/gems/safe_yaml-1.0.4/lib/safe_yaml/psych_resolver.rb:2:in `<module:SafeYAML>'
    from /home/my-user/.gem/ruby/2.2.0/gems/safe_yaml-1.0.4/lib/safe_yaml/psych_resolver.rb:1:in `<top (required)>'
~$ sudo gem update --system

    RubyGems system software updated

~$ gem uninstall psych
    Successfully uninstalled psych-2.0.15

~$ gem install psych -v 2.0.5

    Fetching: psych-2.0.5.gem (100%)
    Building native extensions.  This could take a while...
    Successfully installed psych-2.0.5
    1 gem installed

~$ jekyll serve

    Server address:
    Server running... press ctrl-c to stop.