Ei, eu sou novo em rails (assim como linux) e continuo recebendo este erro:

Installing json (1.7.5) with native extensions 
Gem::Installer::ExtensionBuildError: ERROR: Failed to build gem native extension.

    /usr/bin/ruby1.9.1 extconf.rb 
/usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require': cannot load such file  -- mkmf (LoadError)
from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
from extconf.rb:1:in `<main>'


Gem files will remain installed in /home/jack/.bundler/tmp/10426/gems/json-1.7.5 for inspection.
Results logged to /home/jack/.bundler/tmp/10426/gems/json-1.7.5/ext/json/ext/generator/gem_make.out
An error occurred while installing json (1.7.5), and Bundler cannot continue.
Make sure that `gem install json -v '1.7.5'` succeeds before bundling.
jack@jack-ThinkCentre:~/rails_projects/sample_app$ 

Eu não posso instalar a jóia individualmente ou com a instalação do pacote. Eu usei rvm para instalar os trilhos. desde já, obrigado.