Ubuntuでknife-soloをインストールするときにエラーBuilding native extensions. This could take a while... ERROR: Error installing knife-solo: ERROR: Failed to build gem native extension. /usr/bin/ruby2.1 extconf.rb 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 /var/lib/gems/2.1.0/gems/libyajl2-1.1.0 for inspection. Results logged to /var/lib/gems/2.1.0/extens
