sudo /usr/lib64/fluent/ruby/bin/fluent-gem update

Updating installed gems

Updating io-console
ERROR:  Error installing io-console:
    io-console requires Ruby version >= 2.0.0.

Updating nokogiri
Building native extensions.  This could take a while...
ERROR:  Error installing nokogiri:
    ERROR: Failed to build gem native extension.

        /usr/lib64/fluent/ruby/bin/ruby extconf.rb
checking if the C compiler accepts ... yes
Building nokogiri using packaged libraries.
Using mini_portile version 2.0.0.rc2
checking for gzdopen() in -lz... no
zlib is missing; necessary for building libxml2
*** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of
necessary libraries and/or headers.  Check the mkmf.log file for more
details.  You may need configuration options.

Provided configuration options:
    --with-opt-dir
    --without-opt-dir
    --with-opt-include
    --without-opt-include=${opt-dir}/include
    --with-opt-lib
    --without-opt-lib=${opt-dir}/lib
    --with-make-prog
    --without-make-prog
    --srcdir=.
    --curdir
    --ruby=/usr/lib64/fluent/ruby/bin/ruby
    --help
    --clean
    --use-system-libraries
    --enable-static
    --disable-static
    --with-zlib-dir
    --without-zlib-dir
    --with-zlib-include
    --without-zlib-include=${zlib-dir}/include
    --with-zlib-lib
    --without-zlib-lib=${zlib-dir}/lib
    --enable-cross-build
    --disable-cross-build


Gem files will remain installed in /usr/lib64/fluent/ruby/lib/ruby/gems/1.9.1/gems/nokogiri-1.6.7.2 for inspection.
Results logged to /usr/lib64/fluent/ruby/lib/ruby/gems/1.9.1/gems/nokogiri-1.6.7.2/ext/nokogiri/gem_make.out

Updating ohai
    ERROR:  Error installing ohai:
        ohai requires Ruby version >= 2.0.0.

Nothing to update

ruby -v
ruby 2.0.0p648 (2015-12-16) [x86_64-linux]

fluentdのアップデートを行うと上記のようなエラーがでます。
Rubyのバージョン2.0.0以上が必要とでているのですが、ruby -vで調べると2.0.0p648なので問題ないように思えます。
上記のエラーでは何を要求しているのかがわからないので、教えていただけますか？