If for some reason `Discourse.store.path_for` returns `nil`, the
forum would throw an error rather than returning 404.
Why would it be `nil`? One cause could be changing the type of
file store and having the `url` field no longer be relative.
This updates tests to use latest rails 5 practice
and updates ALL dependencies that could be updated
Performance testing shows that performance has not regressed
if anything it is marginally faster now.
