Include babel-plugin-syntax-object-rest-spread in default Babel config

[novemberborn]

As of 8.3.0, Node.js supports object rest/spread without any flags. Out of the box Babel does not recognize this syntax though. We should include the babel-plugin-syntax-object-rest-spread plugin in our default Babel config.

Note that we only support stage-4 features, and object rest/spread is still stage-3. Thus the question is for which Node.js versions do we include this plugin? I'm leaning towards 8.3.0 and above. This would mean that if you're targeting just those versions you can use object rest/spread with AVA without having to make any customizations. If you're targeting older versions you'll need to configure the full babel-plugin-transform-object-rest-spread, which comes with the syntax plugin.