title: CircleCI gems caching
body: <p>Is it possible to cache gems, so that <code>bundle install</code> will not install bunch of gems for every build? This takes 5 minutes on every build, that is too much.</p>
  
  <p>I've added this to the <code>circle.yml</code> config:</p>
  
  <pre><code>dependencies:
    cache_directories:
      - "/home/ubuntu/.rvm/gems/ruby-2.1.2/gems/"
  </code></pre>
  
  <p>This is the common directory which is provided by <code>bundle show gem_name</code> where all system gems are put.</p>
  
  <p>After this addition system writes such log:</p>
  
  <pre><code>restoring cache v4/company/repo_name/dependency/circle-ci/42/mGWhlYQIxyOy0GZtt4QmCw__.tar.gz
  restoring home/ubuntu/repo_name/vendor/bundle, home/ubuntu/.m2, home/ubuntu/.ivy2, home/ubuntu/.go_workspace, home/ubuntu/.gradle, home/ubuntu/.rvm/gems/ruby-2.1.2/gems
  </code></pre>
  
  <p>So, as far as I understand, it restores system gems and gems installed into <code>vendor/bundle</code>, however, I still see this (and this takes a lot of time): </p>
  
  <pre><code>Installing rake 10.4.2
  Installing i18n 0.7.0
  Installing json 1.8.2
  Installing minitest 5.5.1
  ...
  </code></pre>
  
  <p>instead of </p>
  
  <pre><code>using rake 10.4.2
  using i18n 0.7.0
  using json 1.8.2
  using minitest 5.5.1
  ...
  </code></pre>
  
  <p>So, it seems, it should work, but it does not. What can be wrong?</p>