Eu tive o mesmo erro que você e depois de instalar o gem JSON por sudo gem install json
não tive problemas. link
Instalei o Ruby, o RubyGems e o Jekyll, mas quando executo o Jekyll, estou recebendo este erro.
$ jekyll serve
/var/lib/gems/1.8/gems/commander-4.1.5/lib/commander/runner.rb:365:in 'require_program': program version required (Commander::Runner::CommandError)
from /var/lib/gems/1.8/gems/commander-4.1.5/lib/commander/runner.rb:364:in 'each'
from /var/lib/gems/1.8/gems/commander-4.1.5/lib/commander/runner.rb:364:in 'require_program'
from /var/lib/gems/1.8/gems/commander-4.1.5/lib/commander/runner.rb:52:in 'run!'
from /var/lib/gems/1.8/gems/commander-4.1.5/lib/commander/delegates.rb:7:in 'run!'
from /var/lib/gems/1.8/gems/commander-4.1.5/lib/commander/import.rb:10
from /usr/local/bin/jekyll:19
/usr/lib/ruby/vendor_ruby/1.8/rubygems/custom_require.rb:36:in 'gem_original_require': no such file to load -- json (LoadError)
from /usr/lib/ruby/vendor_ruby/1.8/rubygems/custom_require.rb:36:in 'require'
from /var/lib/gems/1.8/gems/jekyll-1.3.0/bin/../lib/jekyll/filters.rb:2
from /usr/lib/ruby/vendor_ruby/1.8/rubygems/custom_require.rb:36:in 'gem_original_require'
from /usr/lib/ruby/vendor_ruby/1.8/rubygems/custom_require.rb:36:in 'require'
from /var/lib/gems/1.8/gems/jekyll-1.3.0/bin/../lib/jekyll.rb:43
from /usr/lib/ruby/vendor_ruby/1.8/rubygems/custom_require.rb:36:in 'gem_original_require'
from /usr/lib/ruby/vendor_ruby/1.8/rubygems/custom_require.rb:36:in 'require'
from /var/lib/gems/1.8/gems/jekyll-1.3.0/bin/jekyll:7
from /usr/local/bin/jekyll:19:in 'load'
from /usr/local/bin/jekyll:19
somasundaram@somasundaram-Inspiron-3521:~/allfiles/msomu/msomu.github.io$ ^C
somasundaram@somasundaram-Inspiron-3521:~/allfiles/msomu/msomu.github.io$ jekyll/var/lib/gems/1.8/gems/commander-4.1.5/lib/commander/runner.rb:365:in 'require_program': program version required (Commander::Runner::CommandError)
from /var/lib/gems/1.8/gems/commander-4.1.5/lib/commander/runner.rb:364:in 'each'
from /var/lib/gems/1.8/gems/commander-4.1.5/lib/commander/runner.rb:364:in 'require_program'
from /var/lib/gems/1.8/gems/commander-4.1.5/lib/commander/runner.rb:52:in 'run!'
from /var/lib/gems/1.8/gems/commander-4.1.5/lib/commander/delegates.rb:7:in 'run!'
from /var/lib/gems/1.8/gems/commander-4.1.5/lib/commander/import.rb:10
from /usr/local/bin/jekyll:19
/usr/lib/ruby/vendor_ruby/1.8/rubygems/custom_require.rb:36:in 'gem_original_require': no such file to load -- json (LoadError)
from /usr/lib/ruby/vendor_ruby/1.8/rubygems/custom_require.rb:36:in 'require'
from /var/lib/gems/1.8/gems/jekyll-1.3.0/bin/../lib/jekyll/filters.rb:2
from /usr/lib/ruby/vendor_ruby/1.8/rubygems/custom_require.rb:36:in 'gem_original_require'
from /usr/lib/ruby/vendor_ruby/1.8/rubygems/custom_require.rb:36:in 'require'
from /var/lib/gems/1.8/gems/jekyll-1.3.0/bin/../lib/jekyll.rb:43
from /usr/lib/ruby/vendor_ruby/1.8/rubygems/custom_require.rb:36:in 'gem_original_require'
from /usr/lib/ruby/vendor_ruby/1.8/rubygems/custom_require.rb:36:in 'require'
from /var/lib/gems/1.8/gems/jekyll-1.3.0/bin/jekyll:7
from /usr/local/bin/jekyll:19:in 'load'
from /usr/local/bin/jekyll:19
Além disso, se não houver nenhuma maneira de consertá-lo, por favor me ajude se alguém me disser como remover completamente o Ruby, o RubyGems Jekyll e instalá-lo novamente ou corrigir esse erro.