title: Homebrew: list available versions with new formula@version format
body: <p>Homebrew recently deprecated <code>homebrew/versions</code> in favour of making versions available on <code>homebrew/core</code> via the new <code>formula@version</code> format. For example (as per <a href="https://stackoverflow.com/a/9832084/1469018">this answer</a>), you can do <code>brew install postgresql@9.5</code>. </p>
  
  <p>Of course, this doesn't work for arbitrary versions. For instance, <code>install cocoapods@1.1.1</code> turns up "Error: No formulae found in taps".</p>
  
  <p>Under the old method, I could run <code>brew versions &lt;formula&gt;</code> to see available versions. How do I list available versions now?</p>