title: How to install older formula using Brew?
body: <p>Using the case of installing Python 2.7.9 instead of the latest 2.7.10, previously I could simply use <code>brew versions python</code> and see all of the versions of Python formulae by their commit SHA in brew, and then check them out to install a specific version. This is an example of <code>brew versions python</code> output:</p>
  
  <pre><code>$ brew versions python
  Warning: brew-versions is unsupported and will be removed soon.
  You should use the homebrew-versions tap instead:
    https://github.com/Homebrew/homebrew-versions
  2.7.9    git checkout 667284f /usr/local/Library/Formula/python.rb
  2.7.8    git checkout f26ca5c /usr/local/Library/Formula/python.rb
  2.7.7    git checkout d48206e /usr/local/Library/Formula/python.rb
  2.7.6    git checkout 3c64184 /usr/local/Library/Formula/python.rb
  2.7.5    git checkout a04b443 /usr/local/Library/Formula/python.rb
  2.7.3    git checkout 865f763 /usr/local/Library/Formula/python.rb
  2.7.4    git checkout 280581d /usr/local/Library/Formula/python.rb
  2.7.2    git checkout 97c6869 /usr/local/Library/Formula/python.rb
  2.7.1    git checkout 83ed494 /usr/local/Library/Formula/python.rb
  2.7      git checkout 1bf3552 /usr/local/Library/Formula/python.rb
  2.6.5    git checkout acd49f7 /usr/local/Library/Formula/python.rb
  2.6.4    git checkout 843bff9 /usr/local/Library/Formula/python.rb
  2.6.3    git checkout 5c6cc64 /usr/local/Library/Formula/python.rb
  </code></pre>
  
  <p>But the latest version of brew has already removed <code>versions</code> support, and I don't understand how to install a previous Python using <a href="https://github.com/Homebrew/homebrew-versions" rel="noreferrer">https://github.com/Homebrew/homebrew-versions</a>. How do you actually install Python 2.7.9 instead of the newer 2.7.10?</p>
  
  <p>I would like to know how to use <code>homebrew-versions</code> instead of the more trivial way <code>brew versions</code>. </p>
  
  <p>With <code>brew versions</code> I could easily see all versions from that specific formulae (have a look at the above Python versions spit out by <code>brew versions python</code>). From the <a href="https://github.com/Homebrew/homebrew-versions" rel="noreferrer">doc</a>, there is no clear way to achieve the same result as <code>brew versions python</code> does. </p>