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/leonardo/.bundler/tmp/7305/gems/json-1.8.1 for inspection.
Results logged to /home/leonardo/.bundler/tmp/7305/gems/json-1.8.1/ext/json/ext/generator/gem_make.out

An error occurred while installing json (1.8.1), and Bundler cannot continue.
Make sure that `gem install json -v '1.8.1'` succeeds before bundling.