Log Message:
converters/ruby-unf_ext: update to 0.0.8

0.0.8 (2021-09-14)

* No functional change in the library code.
* Include Windows binaries for Ruby 3.0.
* Drop support for Ruby 2.1 and earlier.
* Replace Travis CI with Github Actions.
* Fix cross-build after upgrading rake-compiler/rake-compiler-dock to
  1.1.1/1.1.0.