故障現象：
[root@cloudmaster gem]# gem install rails -l
ERROR: Error installing rails:
activemodel requires builder (~> 3.0.0, runtime)
[root@cloudmaster gem]#
故障分析：
gem包使用本地安裝，而本地builder-3.1.4.gem版本大於3.0.0，按提示應該不是此問題；嘗試其他方法解決都行不通，故找來另一台機器直接gem install rails一下，之後gem list結果如下：
[root@ezCloud ~]# gem install rails
Successfully installed rails-3.2.9
1 gem installed
Installing ri documentation for rails-3.2.9...
Installing RDoc documentation for rails-3.2.9...
[root@ezCloud ~]# gem list
*** LOCAL GEMS ***
actionmailer (3.2.9)
actionpack (3.2.9)
activemodel (3.2.9)
activerecord (3.2.9)
activeresource (3.2.9)
activesupport (3.2.9)
arel (3.0.2)
builder (3.0.4)
bundler (1.2.3)
daemons (1.1.9)
erubis (2.7.0)
eventmachine (1.0.0)
headers (0.0.1)
hike (1.2.1)
i18n (0.6.1)
journey (1.0.4)
json (1.7.5, 1.4.6)
libraries (0.1.6)
mail (2.4.4)
mime-types (1.19)
multi_json (1.5.0)
nokogiri (1.5.6)
polyglot (0.3.3)
rack (1.4.1)
rack-cache (1.2)
rack-protection (1.3.2)
rack-ssl (1.3.2)
rack-test (0.6.2)
rails (3.2.9)
railties (3.2.9)
rake (10.0.3)
rdoc (3.12)
sequel (3.42.0)
sinatra (1.3.3)
sinatra-sequel (0.9.0)
sprockets (2.2.2)
thin (1.5.0)
thor (0.16.0)
tilt (1.3.3)
treetop (1.4.12)
tzinfo (0.3.35)
[root@ezCloud ~]#
