Add metadata to gemspec
Add metadata to gemspec

These are added in template of `bundle gem` since bundler v1.16.6

c.f.

* https://github.com/bundler/bundler/blob/v1.16.6/lib/bundler/templates/newgem/newgem.gemspec.tt#L27-L29
* https://github.com/bundler/bundler/blob/master/CHANGELOG.md#1166-2018-10-05

itamae-kitchen/itamae,sue445/itamae,sue445/itamae,itamae-kitchen/itamae