Gem::Ext::BuildError: ERROR: Failed to build gem native extension.

    /usr/bin/ruby1.9.1 extconf.rb
/usr/local/lib/site_ruby/1.9.1/rubygems/core_ext/kernel_require.rb:55:in `require': cannot load such file -- mkmf (LoadError)
	from /usr/local/lib/site_ruby/1.9.1/rubygems/core_ext/kernel_require.rb:55:in `require'
	from extconf.rb:1:in `<main>'

extconf failed, exit code 1

Gem files will remain installed in /var/lib/gems/1.9.1/gems/json-1.8.1 for inspection.
Results logged to /var/lib/gems/1.9.1/extensions/x86-linux/1.9.1/json-1.8.1/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.