Only use wacky Fiber.yield hack on Ruby
Only use wacky Fiber.yield hack on Ruby

Allegedly this is a fix for an MRI memory leak, but I have not seen
a repro for the problem.

This behavior doesn't make sense and breaks other Ruby VMs, so
constrain it to MRI only.

TiagoCardoso1983/celluloid,kenichi/celluloid,celluloid/celluloid