Replace default hello world AMP document v0.js file

[Gregable]

In validator.ampproject.org, if one selects html format AMP4ADS, the example document uses:

<script async src="https://cdn.ampproject.org/v0.js"></script>

It should use the preferred version of this script:

<script async src="https://cdn.ampproject.org/amp4ads-v0.js"></script>

The same is true in each of our testdata examples in github, testdata/amp4ads_feature_tests/...

[Gregable]

@lannka @jasti