Upgrading GitHub to Rails 3 with Zero Downtime
<p><pre><code>    For those of you keeping score:
    
    - Yes, Rails 3 was released four years ago
    - Yes, the current stable version is Rails 4.1, which left us
      two major versions behind
    
    We had work to do in order to live in the modern world again.
</code></pre>
Okay, so why didn't aren't they transitioning to Rails 4? I'm not clued in to much more than the version numbers so I suppose there are reasons that go a little deeper than 'the lowest version that works with the gems we want'. They've been working on the transition for six months according to the post, making it recent enough that 4 would be the 'obvious' choice unless there were fears that it wouldn't be stable (IIRC Rails 4 has only been so for a few months).