安裝 gem install selenium-webdriver 出現錯誤

Building native extensions.  This could take a while...
ERROR:  Error installing selenium-webdriver:
    ERROR: Failed to build gem native extension.

/usr/bin/ruby extconf.rb
mkmf.rb can't find header files for ruby at /usr/lib/ruby/ruby.h


Gem files will remain installed in /usr/lib/ruby/gems/1.8/gems/ffi-1.9.0 for inspection.
Results logged to /usr/lib/ruby/gems/1.8/gems/ffi-1.9.0/ext/ffi_c/gem_make.out

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solution : yum install ruby-devel