title: Gulp-autoprefixer throwing ReferenceError: Promise is not defined
body: <p>I try to make a gulp compile my sass then autoprefixit with <code>gulp-autoprefixer</code> but i'm getting an error.</p>
  
  <pre><code>var gulp = require('gulp'),
      sass = require('gulp-sass'),
      autoprefixer = require('gulp-autoprefixer');
  
  gulp.task('test', function(){
      gulp.src('_sass/main.sass')
          .pipe(sass())
          .pipe(autoprefixer()) 
          .pipe(gulp.dest('./assets/css')); 
  });
  </code></pre>
  
  <p>I'm trying to run this <code>Gulpfile.js</code> and i'm using :</p>
  
  <pre><code>"gulp": "~3.9.0",
  "gulp-sass": "~2.0.4",
  "gulp-autoprefixer": "~3.0.1",
  </code></pre>
  
  <p>and NPM version <code>1.3.10</code></p>
  
  <p>When i run <code>gulp test</code> i get this :  </p>
  
  <pre><code>/home/matei/Tests/test-4/node_modules/gulp-autoprefixer/node_modules/postcss/lib/lazy-result.js:152
          this.processing = new Promise(function (resolve, reject) {
                                ^
  ReferenceError: Promise is not defined
      at LazyResult.async (/home/matei/Tests/test-4/node_modules/gulp-autoprefixer/node_modules/postcss/lib/lazy-result.js:152:31)
      at LazyResult.then (/home/matei/Tests/test-4/node_modules/gulp-autoprefixer/node_modules/postcss/lib/lazy-result.js:75:21)
      at DestroyableTransform._transform (/home/matei/Tests/test-4/node_modules/gulp-autoprefixer/index.js:28:13)
      at DestroyableTransform.Transform._read (/home/matei/Tests/test-4/node_modules/gulp-autoprefixer/node_modules/through2/node_modules/readable-stream/lib/_stream_transform.js:172:10)
      at DestroyableTransform.Transform._write (/home/matei/Tests/test-4/node_modules/gulp-autoprefixer/node_modules/through2/node_modules/readable-stream/lib/_stream_transform.js:160:12)
      at doWrite (/home/matei/Tests/test-4/node_modules/gulp-autoprefixer/node_modules/through2/node_modules/readable-stream/lib/_stream_writable.js:326:12)
      at writeOrBuffer (/home/matei/Tests/test-4/node_modules/gulp-autoprefixer/node_modules/through2/node_modules/readable-stream/lib/_stream_writable.js:312:5)
      at DestroyableTransform.Writable.write (/home/matei/Tests/test-4/node_modules/gulp-autoprefixer/node_modules/through2/node_modules/readable-stream/lib/_stream_writable.js:239:11)
      at write (/home/matei/Tests/test-4/node_modules/gulp-sass/node_modules/through2/node_modules/readable-stream/lib/_stream_readable.js:623:24)
      at flow (/home/matei/Tests/test-4/node_modules/gulp-sass/node_modules/through2/node_modules/readable-stream/lib/_stream_readable.js:632:7)
  </code></pre>
  
  <p>I don't really know what i'm doing wrong. Is not working when i use sass or plain css. I think is something with my files.</p>