title: ENOENT error when starting serverless offline
body: <p>When running <code>sls offline start</code> I'm getting:</p>
  
  <pre><code>$ sls offline start
  events.js:160
        throw er; // Unhandled 'error' event
        ^
  
  Error: spawn java ENOENT
      at exports._errnoException (util.js:1020:11)
      at Process.ChildProcess._handle.onexit (internal/child_process.js:193:32)
      at onErrorNT (internal/child_process.js:367:16)
      at _combinedTickCallback (internal/process/next_tick.js:80:11)
      at process._tickDomainCallback (internal/process/next_tick.js:128:9)
  </code></pre>
  
  <p>My <code>serverless.yml</code> has:</p>
  
  <pre><code>plugins:
    - serverless-dynamodb-local
    - serverless-offline
    - serverless-mocha-plugin
  </code></pre>