[0.3] Rename parent-hover variant to group-hover

[adamwathan]

Changing a decision from #251.

From this:

<div class="parent p-6 bg-white hover:bg-blue leading-normal font-sans max-w-sm">
  <p class="text-black text-xl font-medium parent-hover:text-white">Heading</p>
  <p class="text-grey-darker parent-hover:text-blue-lighter">Lorem ipsum dolor sit amet, consectetur adipisicing elit. Itaque officia quo praesentium impedit tempore, sapiente, quae eius molestias laborum dicta dolores accusamus. Saepe, omnis. Explicabo suscipit velit, distinctio illum repellendus.</p>
</div>

...to this:

<div class="group p-6 bg-white hover:bg-blue leading-normal font-sans max-w-sm">
  <p class="text-black text-xl font-medium group-hover:text-white">Heading</p>
  <p class="text-grey-darker group-hover:text-blue-lighter">Lorem ipsum dolor sit amet, consectetur adipisicing elit. Itaque officia quo praesentium impedit tempore, sapiente, quae eius molestias laborum dicta dolores accusamus. Saepe, omnis. Explicabo suscipit velit, distinctio illum repellendus.</p>
</div>

I'm not super picky about this change so willing to entertain arguments to keep it as parent if anyone has a really good reason, but my general feeling is that parent makes people think "direct parent" whereas group is a little more decoupled from CSS terminology, so people make less assumptions about it.