Updated to latest babel modules

[mummybot]

When working through the tutorial the latest version of babel has nerfed the "default" module export behaviour.

It requires adding the transform export extensions plugin, otherwise the following will not work:

// core.js
export function setEntries(state, entries) {
  return state.set('entries', entries);
}

//core_spec.js
import {List as list, Map as map} from 'immutable';
import {expect} from 'chai';
import {setEntries} from '../src/core';

describe('application logic', () => {
  describe('setEntries', () => {
    it('adds the entries to the state', () => {
      const entries = list.of('Trainspotting', '28 Days Later');
      const state = map();
      const nextState = setEntries(state, entries); // setEntries is undefined unless the transform export extensions plugin is enabled

      expect(nextState).to.equal(map({
        entries: list.of('Trainspotting', '28 Days Later')
      }));
    });
  });
});