Installing RedCloth (4.2.7)
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 /var/lib/gems/1.9.1/gems/RedCloth-4.2.7 for inspection.
Results logged to /var/lib/gems/1.9.1/gems/RedCloth-4.2.7/ext/redcloth_scan/gem_make.out

An error occurred while installing RedCloth (4.2.7), and Bundler cannot
continue.
Make sure that `gem install RedCloth -v '4.2.7'` succeeds before bundling.