title: gulp autoprefixer doesn't add moz prefix
body: <p>I am using gulp with autoprefixer in my project, and I have to use backgrounds gradient like this:</p>
  
  <pre><code>background: linear-gradient(#e98a00, #f5aa2f);
  </code></pre>
  
  <p>but output is:</p>
  
  <pre><code>background:-webkit-linear-gradient(#e98a00,#f5aa2f);
  background:linear-gradient(#e98a00,#f5aa2f);
  </code></pre>
  
  <p>What wrong with me?</p>
  
  <p><strong>Part of Gulpfile.js</strong></p>
  
  <pre><code>gulp.task('styles', function() {
    return gulp.src(['css/less/mainPage.less'])
      .pipe(plumber())
      // .pipe(concat('base.scss'))
      .pipe(less())
      .pipe(prefix([{ browsers: ['IE 8', 'IE 9', 'last 5 versions', 'Firefox 14', 'Opera 11.1'] }]))
      .pipe(minifyCSS({keepBreaks: true}))
      .pipe(gulp.dest('css'))
      .pipe(connect.reload());
  }); 
  </code></pre>
  
  <p><strong>Iam using gulp-autoprefixer</strong></p>
  
  <p>even if Iam setting </p>
  
  <pre><code> browsers: ['Firefox 14']
  </code></pre>
  
  <p>output still: </p>
  
  <pre><code> background:-webkit-linear-gradient(#e98a00,#f5aa2f);
   background:linear-gradient(#e98a00,#f5aa2f);
  </code></pre>