ERROR: Loading command: install (LoadError)

 gem install rails
ERROR:  Loading command: install (LoadError)
    cannot load such file -- zlib
ERROR:  While executing gem ... (NameError)
    uninitialized constant Gem::Commands::InstallCommand
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yum install zlib-devel 

then in /opt/ruby/ext/zlib/

 ruby extconf.rb
make && make install 

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