Update postcss task

tasks/postcss.js

@@ -1,22 +1,24 @@
 'use strict';
 
-var gulp = require('gulp'),
+const gulp = require('gulp'),
     postcss = require('gulp-postcss'),
     autoprefixer = require('autoprefixer');
 
 function postcssTask(options){
-  gulp.task('postcss', ['dupe'], function () {
+  // Requires: dupe.
+  gulp.task(
+    'postcss',
+    function() {
       var processors = [
-      autoprefixer({
-        browsers: ['last 5 versions']
-      })
+        autoprefixer()
       ];
 
       return options
         .src(options.workingDir + '/**/*.css')
         .pipe(postcss(processors))
         .pipe(gulp.dest(options.workingDir));
-  });
+    }
+  );
 }
 
 module.exports = postcssTask;