Hating on HTML fixtures

The HTML fixtures churn a lot, are a PITA to re-generate, and don't provide a lot of value. Here is an alternate suggestion:

* Store mildly processed HTML that strips out stuff we don't care about.
* Only verify the fixture on modern rubies that all generate the same HTML.
* Older rubies still run a smoke test ensuring the HTML generation does not explode. If they succeed at this, it is highly unlikely that their output will be significantly different from a modern ruby that we _are_ validating the fixture on.

@myronmarston  @JonRowe @soulcutter @samphippen