TwitterのGemを入れようとしたら、エラー吐いた＆その対処法

$ sudo gem install twitter

Building native extensions. This could take a while...
ERROR: Error installing twitter:
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/http_parser.rb-0.6.0 for inspection.
Results logged to /Library/Ruby/Gems/2.0.0/gems/http_parser.rb-0.6.0/ext/ruby_http_parser/gem_make.out

$ xcode-select --install

$ sudo xcodebuild -license