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.
Make it a bit clearer how to use this stuff.
Make tests resilient to changes in Gemfile.
Whoops, if bundle install failed in a way that left no Gemfile.lock, this would silently proceed!

This terminates with a non-zero exit code leaving Bundler's error messages as the last output.
Added Database#timestamp.
Use Dir.chdir.
chdir into the database before running invoking git.
Move the timestamp logic into Database.default.
Renamed Database.default back to path.
Merge remote-tracking branch 'upstream/0.3.0' into 0.3.0
Merge remote-tracking branch 'origin/make_tests_more_resilient' into 0.3.0
Merge remote-tracking branch 'origin/make_readme_more_informative' into 0.3.0
Merge remote-tracking branch 'origin/make_tests_resilient_to_gemfile_changes' into 0.3.0
Updated ruby-advisory-db