title: Sass does not work when i use the foundation 6 for email
body: <p>When i use zurb-foundation-6 for an email template, I found that the command <code>npm run build</code> fails due to</p>
  
  <pre><code>[02:42:06] Failed to load external module @babel/register
  [02:42:06] Requiring external module babel-register
  [02:42:08] Using gulpfile ~/IdeaProjects/edm/gulpfile.babel.js
  [02:42:08] Starting 'default'...
  [02:42:08] Starting 'build'...
  [02:42:08] Starting 'clean'...
  [02:42:08] Finished 'clean' after 5.29 ms
  [02:42:08] Starting 'pages'...
  [02:42:09] Finished 'pages' after 380 ms
  [02:42:09] Starting 'sass'...
  [02:42:09] 'sass' errored after 339 ms
  [02:42:09] TypeError in plugin 'gulp-uncss'
  Message:
      "file" argument must be a non-empty string
  Details:
      domainEmitter: [object Object]
      domain: [object Object]
      domainThrown: false
  [02:42:09] 'build' errored after 726 ms
  [02:42:09] 'default' errored after 728 ms
  npm ERR! code ELIFECYCLE
  npm ERR! errno 1
  npm ERR! foundation-emails-template@1.0.0 build: `gulp --production`
  npm ERR! Exit status 1
  npm ERR! 
  npm ERR! Failed at the foundation-emails-template@1.0.0 build script.
  npm ERR! This is probably not a problem with npm. There is likely 
  additional logging output above.
  
  npm ERR! A complete log of this run can be found in:
  npm ERR!     /Users/baixu/.npm/_logs/2018-05-14T18_42_09_404Z-debug.log
  </code></pre>