title: gulp wont run on gulp watch says [ERR_ASSERTION]: Task function must be specified
body: <p>Please can someone help me, for some reason my gulp watch has stopped working. Here is the versions i am using </p>
  
  <pre><code>Liams-MacBook-Pro-149:pss liamhart$ gulp -v
  [10:17:15] CLI version 2.0.1
  [10:17:15] Local version 4.0.0-alpha.3
  Liams-MacBook-Pro-149:pss liamhart$ npm -v
  6.4.1
  Liams-MacBook-Pro-149:pss liamhart$ node -v
  v8.12.0
  Liams-MacBook-Pro-149:pss liamhart$ gulp
  assert.js:42
  throw new errors.AssertionError({
  ^
  
  AssertionError [ERR_ASSERTION]: Task function must be specified
      at Gulp.set [as _setTask] (/Users/liamhart/Desktop/pss/node_modules/undertaker/lib/set-task.js:10:3)
      at Gulp.task (/Users/liamhart/Desktop/pss/node_modules/undertaker/lib/task.js:13:8)
      at Object.&lt;anonymous&gt; (/Users/liamhart/Desktop/pss/gulpfile.js:55:6)
      at Module._compile (module.js:653:30)
      at Object.Module._extensions..js (module.js:664:10)
      at Module.load (module.js:566:32)
      at tryModuleLoad (module.js:506:12)
      at Function.Module._load (module.js:498:3)
      at Module.require (module.js:597:17)
      at require (internal/module.js:11:18)
  Liams-MacBook-Pro-149:pss liamhart$
  </code></pre>