title: How to specify Gemfile so that it won't install a specific version of a gem
body: <p>I'm new to ruby, and I'm trying to specify a specific version number to <code>rack</code>. I want <code>1.4.5</code> installed and <code>1.5.2</code> removed, and I did 
  <code>gem uninstall rack --version 1.5.2</code>, 
  and specified the version number of <code>rack</code> to <code>1.4.5</code> in file <code>Gemfile.lock</code>.</p>
  
  <p>But after I got up this morning I found that <code>rack 1.5.2</code> was automatically installed back again, and the version of <code>rack</code> in <code>Gemfile.lock</code> was reset to <code>1.5.2</code>. How can I specify a configuration so the machine doesn't automatically do the things above?</p>