Version-bump major version because of integration API change

We previously used `main` inside of `package.json` to define where to
pull in untranspiled code into the project and this change starts using
the `esnext` property as seen in http://2ality.com/2017/06/pkg-esnext.html

That means (to me at least) that any software which has been using this
will have to make changes in order to not break when updating from the
previous versions when their build systems looked for the `main` property.

package.json

@@ -1,6 +1,6 @@
 {
   "name": "notifications-panel",
-  "version": "1.2.7",
+  "version": "2.0.0",
   "description": "The core notifications panel for WordPress.com notifications",
   "esnext": "src/Notifications.jsx",
   "scripts": {