title: extraneous key env is not permitted in circle ci
body: <p>in my <code>circle.yml</code> I added <code>env:</code> header to add my mysql environment variables settings, I saw this in some example online, and it works well and my build pass. but I get this warning:</p>
  
  <p><a href="https://i.stack.imgur.com/jUKve.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/jUKve.png" alt="enter image description here"></a></p>
  
  <p>this is how it looks like in my <code>circle.yml</code></p>
  
  <pre><code>docker:
    - image: elasticsearch:1.3.9
    - image: circleci/mysql:5.6
      env:
        - MYSQL_ALLOW_EMPTY_PASSWORD=true
        - MYSQL_HOST=127.0.0.1
        - MYSQL_ROOT_HOST=%
        - MYSQL_DATABASE=MyDB
  environment:
    SBT_VERSION: 1.1.0
    SBT_OPTS: "-Xms512M -Xmx1536M -Xss1M"
  .....
  </code></pre>
  
  <p>Did anyone had this before?</p>