如何解决 centos6 中 gem install foo 失败的问题

我在centos6.4中通过yum -y install ruby rubygems
安装了ruby和rubygems，然后再通过gem install fpm，发现出错，google了一圈，只找到要更换成taobao源的方案，于是：

# gem sources --remove http://rubygems.org/
# gem sources -a http://ruby.taobao.org/
# gem sources -l
*** CURRENT SOURCES ***

http://ruby.taobao.org/

再然后：
# gem install fpm

出现一下错误：
...
Exception `Errno::EEXIST' at /usr/lib/ruby/1.8/fileutils.rb:243 - File exists - /usr/lib/r
uby/gems/1.8/gems/json-1.8.1/tests
Building native extensions. This could take a while...
Exception `Gem::InstallError' at /usr/lib/ruby/site_ruby/1.8/rubygems/ext/builder.rb:49 -
extconf failed:

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

Exception `Gem::Installer::ExtensionBuildError' at /usr/lib/ruby/site_ruby/1.8/rubygems/in
staller.rb:483 - 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/json-1.8.1 for inspection.
Results logged to /usr/lib/ruby/gems/1.8/gems/json-1.8.1/ext/json/ext/generator/gem_make.o
ut

ERROR: Error installing fpm:
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/json-1.8.1 for inspection.
Results logged to /usr/lib/ruby/gems/1.8/gems/json-1.8.1/ext/json/ext/generator/gem_make.o
ut

怎么解决？我只想装fpm而已。

实际上，我昨天另外一个虚拟机debian7中，是可以直接gem install fpm的，都不需要更改sources。