fix(Button): revert rendering `labeled` buttons as <button>

[levithomason]

This should be fixed by fixing #510, not by creating another patch PR. This bug was introduced in #597.

A labeled button has children that can include a <button> element. Therefore, the container element cannot be a button and also have <button> children. This is not valid markup.

http://technologyadvice.github.io/stardust/elements/button#labeled