error instaling the cocoapods

here is the error: ``` Building native extensions. This could take a while... ERROR: Error installing cocoapods: ERROR: Failed to build gem native extension. /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/bin/ruby extconf.rb mkmf.rb can't find header files for ruby at /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/include/ruby.h Gem files will remain installed in /Library/Ruby/Gems/2.0.0/gems/json-1.8.1 for inspection. Results logged to /Library/Ruby/Gems/2.0.0/gems/json-1.8.1/ext/json/ext/generator/gem_make.out ``` i've search on google for the last 7 hours and i can't find a solution that works for me. i have tried them all Please help me up edit: *after installing the RVM i stil get the same error ``` Building native extensions. This could take a while... ERROR: Error installing cocoapods: ERROR: Failed to build gem native extension. /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/bin/ruby extconf.rb mkmf.rb can't find header files for ruby at /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/include/ruby.h Gem files will remain installed in /Library/Ruby/Gems/2.0.0/gems/json-1.8.1 for inspection. Results logged to /Library/Ruby/Gems/2.0.0/gems/json-1.8.1/ext/json/ext/generator/gem_make.out ``` using this command line : `sudo gem install cocoapods`