我在我的机器上设置 ruby 时遇到问题。我一直在关注“Agile Web Development with Rails”一书，到目前为止一切都很好。
我遇到的问题是当我输入
rails server
我收到以下错误：
/home/myname/.gem/ruby/1.9.1/gems/execjs-1.3.0/lib/execjs/runtimes.rb:50:in `autodetect': Could not find a JavaScript runtime. See https://github.com/sstephenson/execjs for a list of available runtimes. (ExecJS::RuntimeUnavailable)
from /home/myname/.gem/ruby/1.9.1/gems/execjs-1.3.0/lib/execjs.rb:5:in `<module:ExecJS>'
from /home/myname/.gem/ruby/1.9.1/gems/execjs-1.3.0/lib/execjs.rb:4:in `<top (required)>'
from /usr/lib/ruby/gems/1.9.1/gems/coffee-script-2.2.0/lib/coffee_script.rb:1:in `require'
from /usr/lib/ruby/gems/1.9.1/gems/coffee-script-2.2.0/lib/coffee_script.rb:1:in `<top (required)>'
from /usr/lib/ruby/gems/1.9.1/gems/coffee-script-2.2.0/lib/coffee-script.rb:1:in `require'
from /usr/lib/ruby/gems/1.9.1/gems/coffee-script-2.2.0/lib/coffee-script.rb:1:in `<top (required)>'
from /usr/lib/ruby/gems/1.9.1/gems/coffee-rails-3.2.2/lib/coffee-rails.rb:1:in `require'
from /usr/lib/ruby/gems/1.9.1/gems/coffee-rails-3.2.2/lib/coffee-rails.rb:1:in `<top (required)>'
from /home/myname/.gem/ruby/1.9.1/gems/bundler-1.1.0/lib/bundler/runtime.rb:68:in `require'
from /home/myname/.gem/ruby/1.9.1/gems/bundler-1.1.0/lib/bundler/runtime.rb:68:in `block (2 levels) in require'
from /home/myname/.gem/ruby/1.9.1/gems/bundler-1.1.0/lib/bundler/runtime.rb:66:in `each'
from /home/myname/.gem/ruby/1.9.1/gems/bundler-1.1.0/lib/bundler/runtime.rb:66:in `block in require'
from /home/myname/.gem/ruby/1.9.1/gems/bundler-1.1.0/lib/bundler/runtime.rb:55:in `each'
from /home/myname/.gem/ruby/1.9.1/gems/bundler-1.1.0/lib/bundler/runtime.rb:55:in `require'
from /home/myname/.gem/ruby/1.9.1/gems/bundler-1.1.0/lib/bundler.rb:118:in `require'
from /home/myname/Programming/Learning/Learning_Rails/demo/config/application.rb:7:in `<top (required)>'
from /home/myname/.gem/ruby/1.9.1/gems/railties-3.2.2/lib/rails/commands.rb:53:in `require'
from /home/myname/.gem/ruby/1.9.1/gems/railties-3.2.2/lib/rails/commands.rb:53:in `block in <top (required)>'
from /home/myname/.gem/ruby/1.9.1/gems/railties-3.2.2/lib/rails/commands.rb:50:in `tap'
from /home/myname/.gem/ruby/1.9.1/gems/railties-3.2.2/lib/rails/commands.rb:50:in `<top (required)>'
from script/rails:6:in `require'
from script/rails:6:in `<main>'
就是说找不到javascript运行时环境。我去了错误消息推荐的网站，我做了一些谷歌搜索，我读到 execjs 将是一个不错的安装选择。我使用以下命令安装了 execjs：
sudo gem install execjs
sudo gem update
当我运行“gem list”时，我得到以下输出：
*** LOCAL GEMS ***
actionmailer (3.2.2)
actionpack (3.2.2)
activemodel (3.2.2)
activerecord (3.2.2)
activeresource (3.2.2)
activesupport (3.2.2)
arel (3.0.2)
bigdecimal (1.1.0)
builder (3.0.0)
bundler (1.1.0)
coffee-rails (3.2.2)
coffee-script (2.2.0)
coffee-script-source (1.2.0)
erubis (2.7.0)
execjs (1.3.0 ruby)
hike (1.2.1)
i18n (0.6.0)
io-console (0.3)
journey (1.0.3)
jquery-rails (2.0.1)
json (1.6.5, 1.5.4)
mail (2.4.3)
mime-types (1.17.2)
minitest (2.11.3, 2.5.1)
multi_json (1.1.0)
polyglot (0.3.3)
rack (1.4.1)
rack-cache (1.2)
rack-ssl (1.3.2)
rack-test (0.6.1)
rails (3.2.2)
railties (3.2.2)
rake (0.9.2.2)
rdoc (3.12, 3.9.4)
sass (3.1.15)
sass-rails (3.2.4)
sprockets (2.3.1, 2.1.2)
sqlite3 (1.3.5)
thor (0.14.6)
tilt (1.3.3)
treetop (1.4.10)
tzinfo (0.3.32)
uglifier (1.2.3)
从上面的输出看，我觉得 execjs 已经安装了。这是怎么回事，为什么我会收到这个错误？
如果有帮助的话，
我的操作系统是 Arch Linux。
ruby -v：ruby 1.9.3p125（2012-02-16 修订版 34643）[x86_64-linux]
导轨 -v：导轨 3.2.2
任何帮助，将不胜感激。