Installation Getting Stuck with json install error.

I'm sorry if this too basic...

 

I'm trying to install wagn on an Ubuntu 12.04 server.  I'm on step two of the installation guide (http://wagn.org/installation), where I'm trying the command 

 

bundle install --without postgres:memcache:test:debug:development:assets

 

The install begins, but then stops with this information:

 

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/michael/.bundler/tmp/10545/gems/json-1.7.7 for inspection.

Results logged to /home/michael/.bundler/tmp/10545/gems/json-1.7.7/ext/json/ext/generator/gem_make.out

 

An error occurred while installing json (1.7.7), and Bundler cannot continue.

Make sure that `gem install json -v '1.7.7'` succeeds before bundling.

 

So, I try gem install json -v '1.7.7' and get this:

 

Building native extensions.  This could take a while...

ERROR:  Error installing json:

        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/json-1.7.7 for inspection.

Results logged to /var/lib/gems/1.9.1/gems/json-1.7.7/ext/json/ext/generator/gem_make.out

 

Please note, the same result happens if I use sudo. I'm over my head here, and Google indicates this is a known problem, but with no real solutions. Does anyone know how I can resolve this problem and move on? 

 

Thanks,

Michael