title: ruby script/rails server fail
body: <p>I am new to the ruby world.
  I want to run an existing Rails project, but I get the following error:</p>
  
  <pre><code>$ruby script/rails server
  /Users/apple/.rvm/gems/ruby-1.9.3-p194/gems/json-1.7.0/ext/json/ext/json/ext/parser.bundle: undefined class/module Encoding (ArgumentError)
      from /Users/apple/.rvm/gems/ruby-1.9.3-p194/gems/activesupport-3.2.1/lib/active_support/dependencies.rb:251:in `require'
      from /Users/apple/.rvm/gems/ruby-1.9.3-p194/gems/activesupport-3.2.1/lib/active_support/dependencies.rb:236:in `load_dependency'
  </code></pre>
  
  <p>Should I install other packages or another version of rails?
  If I write a small project using  <code>rails new xxx</code> then <code>$ruby script/rails server</code> works. </p>
  
  <p>BTW: I am in mac.</p>
  
  <p>Thank you!</p>