title: How to `bundle install` when your Gemfile requires an older version of bundler?
body: <p>I am in an older Rails project that has a Gemfile. I tried to add a gem to the Gemfile and <code>bundle install</code> but got an error:</p>
  
  <pre><code>Bundler could not find compatible versions for gem "bundler":
    In Gemfile:
      rails (= 3.0.0) ruby depends on
        bundler (~&gt; 1.0.0) ruby
  
    Current Bundler version:
      bundler (1.1.5)
  
  This Gemfile requires a different version of Bundler.
  </code></pre>
  
  <p>The version of Rails it's using requires bundler ~>1.0.0 but I have 1.1.5 installed and am using it for my other projects. Usually I would use <code>bundle exec ...</code> but since this is bundler we are talking about, it's a little more complicated than that. How can I add a gem to my Gemfile and run <code>bundle install</code> while using the version of bundler that it requires?</p>