Register Ruby and LGPLv3 licenses separately

[joshcooper]

Updates the gemspec to register each license using the plural licenses= setter[1] and the respective SPDX ID, so that it's machine readable.

[1] https://guides.rubygems.org/specification-reference/#licenses=

[joshcooper]

❯ bundle exec gem build gettext                           
...
  Successfully built RubyGem
  Name: gettext
  Version: 3.4.1
  File: gettext-3.4.1.gem

❯ bundle exec gem specification gettext-3.4.1.gem | grep -A2 licenses
licenses:
- Ruby
- LGPL-3.0+

[kou]

Good catch!

[joshcooper]

Thanks @kou! Would it be possible to release a gem?

[kou]

Sure. Done.