Change `search_text` to use `option.text` rather than `option.html`

Ember.js litters the DOM with `<script>` tags that are used to perform its DOM binding. So when using an `Ember.Select` component with Chosen, searches would never return results because the regex binds to the start using `/^...`, and the value of `option.html` always starts with a `<script>` tag - so it would never match the regex.

This PR fixes the problem by searching on `option.text`. Another solution would be to change the regex to not bind to the start, but these seemed to be a less significant change.