Make tests more resilient to updates in vulnerability data.

This should cause tests to fail when the format changes, but prevent
failures simply because a previously-not-known-to-be-vulnerable
version had a vulnerability discovered in it.

The caveat is that the `secure` bundle will still kvetch when one of
its gems comes up vulnerable, since it's expected to have ZERO
matches.  In that case, the relevant Gemfile/Gemfile.lock need to be
updated.
Update the secure bundle to reference versions that don't have known vulns.
DRY up a bit.

While this code is more complex, it doesn't duplicate logic from
Bundler::Audit::Advisory.
Merge pull request #52 from MrJoy/make_tests_more_resilient2

Make tests more resilient to changes in details of the output from `bundler-audit`.