Replaced deprecated requireing of yml file

When using `gulp-imagemin` I got the following error:

```
.../node_modules/gulp-imagemin/node_modules/imagemin/node_modules/imagemin-svgo/node_modules/svgo/lib/svgo/config.js:58
    return plugins.map(function(item) {
                   ^
TypeError: Cannot call method 'map' of undefined
```

My project also uses a more recent version of `js-yaml` which conflicted with the version used in svgo.  
So, I replaced the deprecated `require` as described in https://github.com/nodeca/js-yaml#breaking-changes-in-2xx---3xx and updated `js-yaml`.

Should also fix #245.