title: Travis-CI: how to cache composer executable between builds?
body: <p>On every travis build, <code>composer self-update</code> is run. And it gets updated on every build. Is it possible to cache composer executables like we do it with <code>vendor</code> dir via</p>
  
  <pre><code>cache:
    directories:
      - vendor
      - $HOME/.composer/cache
  </code></pre>
  
  <p>I thought about caching the whole <code>/home/travis/.phpenv/versions/5.5/bin/composer</code> but I feel this is not right because the contents of this folder may change without notifying caching system about a change (when travis updates php version for example).</p>
  
  <p>Any suggestions (except for custom composer, of course)?</p>