Support Ruby/GIO2 2.2.0 or later

Ruby/GIO2 2.2.0 (that is not released yet) will be based on Ruby/GObjectIntrospection.
And Ruby/GIO2 API will be changed.

This pull request is for supporting Ruby/GIO2 2.2.0. Could you confirm it after Ruby/GIO2 2.2.0 is released?