Ruby filter

Changes `:ruby` filter to not redirect `$stdout`.

See #613, #614, #616.

I think this is the only way to fix this, I can’t see any way to safely redirect `$stdout` for a single thread (without replacing it with a custom IO of some sort).

If we do go with this, do we need to provide an option to leave the old behaviour for people running single-threaded, and totally remove it in the next version, or make a clean switch in this version? If so, what should the default be?