title: Xcode: What is LastUpgradeCheck?
body: <p>I did a git diff and saw in <code>Project.xcodeproj/project.pbxproj</code>:</p>
  
  <pre><code>    attributes = {
              BuildIndependentTargetsInParallel = YES;
  +           LastUpgradeCheck = 0420;
      };
  </code></pre>
  
  <p>What's <code>LastUpgradeCheck</code>? Maybe I accidentally opened up the project with Xcode 4.2?</p>