Script to regenerate HTML fixtures.
Store canonical HTML for fixtures that excludes most of the backtraces.

This means less churn in diffs. We weren't asserting on these lines
anyway.
Remove rbx suffix for fixtures, it matches CRuby now.
Only verify HTML fixtures on modern rubies.

This still gives us a high level of confidence in the HTML formatter,
while drastically reducing the complexity of the specs. We only require
a single fixture with this method.
Merge pull request #1306 from rspec/hating-on-html-fixtures

Hating on HTML fixtures