Fix Ruby binding building

[KitaitiMakoto]

Hello,

I improved the way of building Ruby binding.

• C/C++ files required to build binding are updated
• You can build gem file using Rake: rake build
• You can run tests using Rake: rake test
• You can run install the gem in your local machine: rake install
• You can release the gem by rake release:rubygem_push
• Test on CI succeeds again

Can you consider this pull request?

Thanks.