Json native gem requires installed build tools

Let's fix this :

$> gem install activesupport
Fetching: i18n-0.7.0.gem (100%)
Fetching: json-1.8.3.gem (100%)
ERROR: Error installing activesupport:
The 'json' native gem requires installed build tools.

Please update your PATH to include build tools or download the DevKit
from 'http://rubyinstaller.org/downloads' and follow the instructions
at 'http://github.com/oneclick/rubyinstaller/wiki/Development-Kit'

Solution

Downlaod the DevKit at http://rubyinstaller.org/downloads/

Extract it to a permanant place of your choice. I chose : C:\Ruby193\DevKit

$> cd C:\Ruby193\DevKit
$> ruby dk.rb init
$> ruby dk.rb install
[INFO] Updating convenience notice gem override for 'C:/Ruby193'
[INFO] Installing 'C:/Ruby193/lib/ruby/site_ruby/devkit.rb'

Et voilà! You can gem install activesupport