title: Two versions of rack in Gemfile.lock causing error
body: <p>Rails app runs fine locally but on the remote server I get this error:</p>
  
  <blockquote>
    <p>You have already activated rack 1.4.1, but your Gemfile requires rack 1.4.4. Using bundle exec may solve this.</p>
  </blockquote>
  
  <p>I've <code>bundle update</code>-ed several times.</p>
  
  <p>My Gemfile.lock has these lines (<code>...</code> indicates removed lines for clarity):</p>
  
  <pre><code>...
  actionpack (3.2.11)
    ...
    rack (~&gt; 1.4.0)
  ...
  rack (1.4.4)
  ...
  </code></pre>
  
  <p>Gemfile:</p>
  
  <pre><code>source 'http://rubygems.org'
  
  gem 'rails', '3.2.11'
  gem "haml"
  gem 'execjs'
  gem "rest-client"
  gem "stripe"
  
  group :assets do
    gem 'sass-rails', "~&gt; 3.2.4"
    gem 'coffee-rails'
    gem 'uglifier'
    gem 'compass-rails'
    # Deploy with Capistrano
    gem 'capistrano'
    gem 'rvm-capistrano'
  end
  
  gem 'jquery-rails'
  gem 'jquery-ui-rails'
  
  group :production do
    gem 'libv8'
    gem 'therubyracer', "~&gt; 0.10.2"
    gem "newrelic_rpm"
    gem 'exception_notification', git: 'git://github.com/alanjds/exception_notification.git', :require =&gt; 'exception_notifier'
  end
  
  gem 'sqlite3' # This has to be active in both modes
  
  group :development do
    gem "thin"
  end
  
  group :test do
    # Pretty printed test output
    gem 'turn', :require =&gt; false
  end
  </code></pre>