$ gem install json -v '1.8.3'
Building native extensions.  This could take a while...
ERROR:  Error installing json:
        ERROR: Failed to build gem native extension.

    /usr/bin/ruby -r ./siteconf20150701-7344-1kh8kia.rb extconf.rb
mkmf.rb can't find header files for ruby at /usr/share/include/ruby.h

extconf failed, exit code 1

Gem files will remain installed in /home/fadmin/.gem/ruby/gems/json-1.8.3 for inspection.
Results logged to /home/fadmin/.gem/ruby/extensions/x86-linux/json-1.8.3/gem_make.out