title: lerna run --parallel not working for rollup watch
body: <h1>Background:</h1>
  
  <p>I have a lerna monorepo with yarn workspaces with two packages. I am using rollup as the bundler.</p>
  
  <p>packages/module1/package.json:</p>
  
  <pre><code>{
    scripts: {
      "watch": "rollup -c rollup.config.js --watch",
      "build": "NODE_ENV=production &amp;&amp; rollup -c rollup.config.js"
    }
  }
  </code></pre>
  
  <p>packages/module2/package.json:</p>
  
  <pre><code>{
    scripts: {
      "watch": "rollup -c rollup.config.js --watch",
      "build": "NODE_ENV=production &amp;&amp; rollup -c rollup.config.js"
    }
  }
  </code></pre>
  
  <h1>Expected Behavior:</h1>
  
  <ol>
  <li><code>lerna run build</code> will run the <code>build</code> scripts for each package.</li>
  <li><code>lerna run watch</code> will run the <code>watch</code> scripts for each package in watch mode. </li>
  </ol>
  
  <h1>Current Behavior:</h1>
  
  <ol>
  <li><code>lerna run build</code> works as expected. The <code>build</code> script runs properly for both packages.</li>
  <li><code>lerna run watch</code> just hangs there:</li>
  </ol>
  
  <pre><code>lerna notice cli v3.13.1
  lerna info Executing command in 2 packages: "yarn run watch"
  [[just hangs here]]
  </code></pre>
  
  <p>I have tried <code>lerna run --parallel watch</code>, and <em>this only runs once</em>. It exits after rollup completes. In other words, it never seems to be watching.</p>