Revert "Drop 1.8.7 and ree support."
Revert "Drop 1.8.7 and ree support."

This reverts commit f0c910ab0c3142210f53cbe637e05f847374f6a3.

knu/ruby-unf_ext,knu/ruby-unf_ext,allenh1/ruby-unf_ext,allenh1/ruby-unf_ext