Autoprefixing in Gulp causes errors that block style.css compile.

The following error shows upon running `gulp styles`:

``` events.js:72
        throw er; // Unhandled 'error' event
              ^
CssSyntaxError: /Users/kwight/Sites/trunk/src/wp-content/themes/picard/style.css.map:3:1: Unknown word
```

[Commenting out autoprefixer from the equation](https://github.com/Automattic/Picard/blob/master/gulpfile.js#L62) allows the compile to complete. 
