mrbgem.rake: Use mruby-metaprog
mrbgem.rake: Use mruby-metaprog

`define_singleton_method` has been moved to that built-in mrbgem.

https://github.com/mruby/mruby/commit/e471d37ca5f1422860a1eaa81d4c9f1b3c8b6aed#diff-11a40737f4b1879f0248d5da1439468d3905879c42e1e347a32d3b68771d1d8d

Asmod4n/mruby-zmq,Asmod4n/mruby-zmq