ruby-on-rails - rails_admin:install を生成できません

Question

bundle exec rails generate rails_admin:install

/home/vieirater/.rvm/gems/ruby-2.0.0-p247/gems/nokogiri-1.6.0/lib/nokogiri.rb:28:in `require': cannot load such file -- nokogiri/nokogiri (LoadError)
    from /home/vieirater/.rvm/gems/ruby-2.0.0-p247/gems/nokogiri-1.6.0/lib/nokogiri.rb:28:in `<top (required)>'
    from /home/vieirater/.rvm/gems/ruby-2.0.0-p247/gems/rack-pjax-0.7.0/lib/rack/pjax.rb:1:in `require'
    from /home/vieirater/.rvm/gems/ruby-2.0.0-p247/gems/rack-pjax-0.7.0/lib/rack/pjax.rb:1:in `<top (required)>'
    from /home/vieirater/.rvm/gems/ruby-2.0.0-p247/gems/rack-pjax-0.7.0/lib/rack-pjax.rb:2:in `require'
    from /home/vieirater/.rvm/gems/ruby-2.0.0-p247/gems/rack-pjax-0.7.0/lib/rack-pjax.rb:2:in `<top (required)>'
    from /home/vieirater/.rvm/gems/ruby-2.0.0-p247/gems/rails_admin-0.5.0/lib/rails_admin/engine.rb:7:in `require'
    from /home/vieirater/.rvm/gems/ruby-2.0.0-p247/gems/rails_admin-0.5.0/lib/rails_admin/engine.rb:7:in `<top (required)>'
    from /home/vieirater/.rvm/gems/ruby-2.0.0-p247/gems/rails_admin-0.5.0/lib/rails_admin.rb:1:in `require'
    from /home/vieirater/.rvm/gems/ruby-2.0.0-p247/gems/rails_admin-0.5.0/lib/rails_admin.rb:1:in `<top (required)>'
    from /home/vieirater/.rvm/gems/ruby-2.0.0-p247@global/gems/bundler-1.3.5/lib/bundler/runtime.rb:72:in `require'
    from /home/vieirater/.rvm/gems/ruby-2.0.0-p247@global/gems/bundler-1.3.5/lib/bundler/runtime.rb:72:in `block (2 levels) in require'
    from /home/vieirater/.rvm/gems/ruby-2.0.0-p247@global/gems/bundler-1.3.5/lib/bundler/runtime.rb:70:in `each'
    from /home/vieirater/.rvm/gems/ruby-2.0.0-p247@global/gems/bundler-1.3.5/lib/bundler/runtime.rb:70:in `block in require'
    from /home/vieirater/.rvm/gems/ruby-2.0.0-p247@global/gems/bundler-1.3.5/lib/bundler/runtime.rb:59:in `each'
    from /home/vieirater/.rvm/gems/ruby-2.0.0-p247@global/gems/bundler-1.3.5/lib/bundler/runtime.rb:59:in `require'
    from /home/vieirater/.rvm/gems/ruby-2.0.0-p247@global/gems/bundler-1.3.5/lib/bundler.rb:132:in `require'
    from /home/vieirater/University/config/application.rb:7:in `<top (required)>'
    from /home/vieirater/.rvm/gems/ruby-2.0.0-p247/gems/railties-4.0.0/lib/rails/commands.rb:44:in `require'
    from /home/vieirater/.rvm/gems/ruby-2.0.0-p247/gems/railties-4.0.0/lib/rails/commands.rb:44:in `<top (required)>'
    from bin/rails:4:in `require'
    from bin/rails:4:in `<main>'