title: How do you fix this Heroku error with the pg gemfile and Rails 3.1?
body: <p>I'm trying to upload a Rails 3.1 app (with CoffeeScript) to Heroku.  Apparently, there are known issues with this
  (http://stackoverflow.com/questions/6075961/problem-deploying-rails-3-1-project-to-heroku-could-not-find-a-javascript-runtim), so I added this to my Gemfile:</p>
  
  <pre><code>group :production do
    gem 'therubyracer-heroku', '0.8.1.pre3'
  end
  </code></pre>
  
  <p>Then, after a few more messing around, I found out I had to do the following:</p>
  
  <pre><code>group :production do
    gem 'therubyracer-heroku', '0.8.1.pre3'
    gem 'pg'
    # pg from http://stackoverflow.com/questions/6410623/heroku-error-when-launch-rails3-1-app-missing-postgres-gem
  end
  </code></pre>
  
  <p>Now I hit this error: "You have modified your Gemfile in development but did not check the resulting snapshot (Gemfile.lock) into version control"</p>
  
  <pre><code>git push heroku master
  Counting objects: 5, done.
  Delta compression using up to 4 threads.
  Compressing objects: 100% (3/3), done.
  Writing objects: 100% (3/3), 402 bytes, done.
  Total 3 (delta 2), reused 0 (delta 0)
  
  -----&gt; Heroku receiving push
  -----&gt; Rails app detected
  -----&gt; Detected Rails is not set to serve static_assets
         Installing rails3_serve_static_assets... done
  -----&gt; Configure Rails 3 to disable x-sendfile
         Installing rails3_disable_x_sendfile... done
  -----&gt; Configure Rails to log to stdout
         Installing rails_log_stdout... done
  -----&gt; Gemfile detected, running Bundler version 1.0.7
         Unresolved dependencies detected; Installing...
         Using --without development:test
         You have modified your Gemfile in development but did not check
         the resulting snapshot (Gemfile.lock) into version control
  
         You have added to the Gemfile:
         * pg
         FAILED: http://devcenter.heroku.com/articles/bundler
   !     Heroku push rejected, failed to install gems via Bundler
  </code></pre>
  
  <p>Well, there is no updated Gemfile.lock to commit... even after I run bundle update.  What's going on?  How the heck do I deploy to Heroku?  I thought Ruby on Rails and Heroku were supposed to be easy to get started with!</p>