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.

Start by running `gulp styles` and inspect the autoprefixer configuration at gulpfile.js line 62. Compare the failing run with autoprefixer commented out, then trace why style.css.map is being parsed as CSS and confirm that the style.css compilation completes without the reported CssSyntaxError.

Written by the indexing model from the issue text.