Use CommonJS until Babel ES3 transforms are fixed

Our current presets didn't include ES3 transforms by default, so IE8 support is broken. Normally we would have fixed it by adding a few plugins:

* babel-plugin-transform-es3-member-expression-literals
* babel-plugin-transform-es3-property-literals

Unfortunately there is a [bug in Babel](https://phabricator.babeljs.io/T2817) that prevents them from working correctly in certain cases with `export default` declarations. Until this is resolved, we will go back to using CommonJS modules internally.

src/index.js

@@ -1,6 +1,8 @@
-export default function thunkMiddleware({ dispatch, getState }) {
+function thunkMiddleware({ dispatch, getState }) {
   return next => action =>
     typeof action === 'function' ?
       action(dispatch, getState) :
       next(action);
 }
+
+module.exports = thunkMiddleware