Convert CommonJS exports to ES6-compatible exports

We use `export default` begrudgingly here. Ideally we'd use just `export`, though this entire SDK expects things to be exported as a default. Instead of breaking everything, we'll sacrifice our export pattern for a smaller diff - a later commit can always do the default export -> regular export conversion.

src/stores/RoomViewStore.js

@@ -1,6 +1,7 @@
 /*
 Copyright 2017 Vector Creations Ltd
 Copyright 2017, 2018 New Vector Ltd
+Copyright 2019 The Matrix.org Foundation C.I.C.
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.
@@ -357,4 +358,4 @@ let singletonRoomViewStore = null;
 if (!singletonRoomViewStore) {
     singletonRoomViewStore = new RoomViewStore();
 }
-module.exports = singletonRoomViewStore;
+export default singletonRoomViewStore;