title: PostCSS error: [object Object] is not a PostCSS plugin
body: <p>The error is coming from the <code>postcss</code> plugin, I think I may have written it incorrectly. </p>
  
  <p>I'm trying to add <code>cssnano</code> and <code>autoprefixer</code> to the <code>postcss</code> plugin.</p>
  
  <pre><code>gulp/node_modules/gulp-postcss/node_modules/postcss/lib/processor.js:143
          throw new Error(i + ' is not a PostCSS plugin');
          ^
  
  Error: [object Object] is not a PostCSS plugin
      at Processor.normalize (/Applications/XAMPP/xamppfiles/htdocs/sites/gulp/node_modules/gulp-postcss/node_modules/postcss/lib/processor.js:143:15)
      at new Processor (/Applications/XAMPP/xamppfiles/htdocs/sites/gulp/node_modules/gulp-postcss/node_modules/postcss/lib/processor.js:51:25)
      at postcss (/Applications/XAMPP/xamppfiles/htdocs/sites/gulp/node_modules/gulp-postcss/node_modules/postcss/lib/postcss.js:73:10)
      at Transform.stream._transform (/Applications/XAMPP/xamppfiles/htdocs/sites/gulp/node_modules/gulp-postcss/index.js:47:5)
      at Transform._read (_stream_transform.js:167:10)
      at Transform._write (_stream_transform.js:155:12)
      at doWrite (_stream_writable.js:300:12)
      at writeOrBuffer (_stream_writable.js:286:5)
      at Transform.Writable.write (_stream_writable.js:214:11)
      at DestroyableTransform.ondata (/Applications/XAMPP/xamppfiles/htdocs/sites/gulp/node_modules/gulp-sass/node_modules/through2/node_modules/readable-stream/lib/_stream_readable.js:531:20)
  Mac-a45e60e72dad:gulp JoeKonst$ 
  </code></pre>
  
  <p>My code:</p>
  
  <pre><code>// Dependancies
  var gulp = require('gulp'),
      browserSync = require('browser-sync'),
      plumber = require('gulp-plumber'),
      autoprefixer = require('gulp-autoprefixer'),
      uglify = require('gulp-uglify'),
      compass = require('gulp-compass'),
      rename = require('gulp-rename'),
      nano = require('cssnano'),
      del = require('del'),
      postcss = require('gulp-postcss'),
      sass = require('gulp-sass');
  
  // Styles
  gulp.task('styles', function(){
      gulp.src('sass/main.scss')
      .pipe(sass())
      .pipe(postcss([autoprefixer({browsers: ['last 2 versions']}), nano()]))
      .pipe(gulp.dest('css/'));
  
      gulp.watch('sass/**/*.scss', ['styles']);
  });
  
  // Tasks
  gulp.task('default', ['styles']);
  </code></pre>