[4] pry(main)> gem-install jist
Building native extensions.  This could take a while...
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/john/.gem/ruby/1.9.1/gems/json-1.7.6 for inspection.
Results logged to /home/john/.gem/ruby/1.9.1/gems/json-1.7.6/ext/json/ext/generator/gem_make.out
from /usr/lib/ruby/1.9.1/rubygems/installer.rb:556:in `rescue in block in build_extensions'