title: Vapor build failure
body: <p>I am using Mac with Sierra OS (10.12.5). I Installed Vapor using home-brew. If I tried to run command 'vapor build' in terminal it shows following error.</p>
  
  <pre><code>No .build folder, fetch may take a while...
  Fetching Dependencies [  •                      ]
  Error: backgroundExecute(code: 1, error: "swift-package: error: unknown command: --enable-prefetching\n", output: "")
  </code></pre>
  
  <p>Please help me.</p>