`Fetching: json-1.8.3.gem (100%)
Building native extensions. This could take a while...
ERROR: Error installing json:
ERROR: Failed to build gem native extension.
mkmf.rb can't find header files for ruby at /usr/lib/ruby/include/ruby.h
extconf failed, exit code 1
Gem files will remain installed in /usr/lib/ruby/gems/2.2.0/gems/json-1.8.3 for inspection.
Results logged to /usr/lib/ruby/gems/2.2.0/extensions/x86_64-linux/2.2.0/json-1.8.3/gem_make.out`
`Fetching: json-1.8.3.gem (100%) Building native extensions. This could take a while... ERROR: Error installing json: ERROR: Failed to build gem native extension.
mkmf.rb can't find header files for ruby at /usr/lib/ruby/include/ruby.h
extconf failed, exit code 1
Gem files will remain installed in /usr/lib/ruby/gems/2.2.0/gems/json-1.8.3 for inspection. Results logged to /usr/lib/ruby/gems/2.2.0/extensions/x86_64-linux/2.2.0/json-1.8.3/gem_make.out`
How can we fix this?