This PR adds license to gemspec.
It is friendly to write license because people can know how this gem are permitted to use.
Also describing LICENSES on rubygems.org (https://rubygems.org/gems/romaji)
Coverage remained the same at 100.0% when pulling 56dbb86065edfa121ee5cd7241c0493bd8eef198 on hisas:add_license_to_gemspec into 1820d641cf5fe72a17c2432059b33e475ebe92c1 on makimoto:master.
This PR adds license to gemspec. It is friendly to write license because people can know how this gem are permitted to use. Also describing LICENSES on rubygems.org (https://rubygems.org/gems/romaji)