Fix trailing comma from previous PR merge

Turns out this is actually illegal in ES6 but on some version of babel,
they don’t care.

Might be an ES7 thing though:
http://www.2ality.com/2015/11/trailing-comma-parameters.html

js/components/register_piece.js

@@ -44,7 +44,7 @@ let RegisterPiece = React.createClass( {
         return mergeOptions(
             UserStore.getState(),
             WhitelabelStore.getState(),
-            PieceListStore.getState(),
+            PieceListStore.getState()
         );
     },