sound_cloud.js: Get the `<script>` tag in the callback.

We should get the `<script>` tag in the callback function since the call to `/js/spice/sound_cloud/` might not exist yet when `sound_cloud.js` loads.
Merge pull request #947 from duckduckgo/jag/soundcloud-fix

sound_cloud.js: Get the `<script>` tag in the callback.