Do not depend on queue_classic on JRuby

Before this commit Rails failed to bundle on JRuby:
https://travis-ci.org/rails/rails/jobs/32948994#L268

Reason: https://github.com/ryandotsmith/queue_classic depends on `pg` which is not supported on JRuby.
The replacement https://github.com/bdon/queue_classic_java seems to be dead and has not been released yet.

After this commit the tests run (with failures) on Travis:
https://travis-ci.org/rails/rails/jobs/32964951