Use ember-auto-import instead of ember-browserify

ember-auto-import is used instead of ember-browserify to import yadda.

💥 This is a breaking change.

To upgrade from 0.4.0:

1. npm install ember-cli-yadda@0.5.0 --save-dev
2. npm uninstall ember-browserify
3. In the application test code, replace every instance of:
   import yadda from '../../helpers/yadda';
   with
   import { yadda } from 'ember-cli-yadda';
4. Delete tests/helpers/yadda.js
5. Run tests