title: Travis not caching directory
body: <p>I've got a .travis.yml file that is describing the directory to cache, however when I check the cache dropdown in travis, it tells me there is nothing. I'm just trying to cache my composer vendor folder. Below is my .travis.yml file:</p>
  
  <pre><code>sudo: required
  
  language: php
  
  php:
    - 7.0
  
  services:
    - docker
  
  before_install:
    - docker-compose up -d
  
  install: composer install
  cache:
    directories:
      - root/vendor
  
  script:
    - bundle exec rake phpcs
    - bundle exec rake phpunit:run
    - bundle exec rake ci:behat
  </code></pre>
  
  <p>And this is my project structure (or the folders/files that matter):</p>
  
  <pre><code>  |-- .travis.yml
    |-- root
      |-- vendor
  </code></pre>
  
  <p>Any suggestions as to why this would be the case?</p>