Drop git ls-files in gemspec

[utkarsh2102]

Hi @arnau,

Thanks for working on this! ❤️
However, while maintaining this in Debian, we found that this library relies on git to list the files which could be done via pure Ruby alternative -- which is what this PR does.

As an addition, this PR makes sure that this gem only ships the required files to the end users and not other things which are not needed by them! 🚀

Also, added rubocop-packaging as a development_dependency which will ensure the best practices.
Here's what it shows us:

➜  ISO8601 git:(master)  rubocop --only Packaging

Offenses:

iso8601.gemspec:19:54: C: Packaging/GemspecGit: Avoid using git to produce lists of files. Downstreams
often need to build your package in an environment that does not have git (on purpose). Use some pure
Ruby alternative, like Dir or Dir.glob.
  s.files =  %w{LICENSE README.md CONTRIBUTING.md} + `git ls-files`.split("\n").select { |f| f =~ %r{^(?:lib/)}i }
                                                     ^^^^^^^^^^^^^^

31 files inspected, 1 offense detected

And this PR fixes the same, which helps us in shipping this in Debian! 🎉
Hope this would make sense and you'll be open to this change 💯

Signed-off-by: Utkarsh Gupta <utkarsh@debian.org>

[arnau]

Thanks for this @utkarsh2102, always nice to remove dependencies :)

I'll release it shortly.

[utkarsh2102]

Aha, I see you're looking for help (and possibly for a new maintainer).
Perhaps, I can at least, help in keeping things updated and take care of your baby in the best way I can! 😄

[utkarsh2102]

There are some more general RuboCop offenses I see. D'you want me to possibly raise yet another PR for that?
(and you can pick the things you want to keep! :))

[arnau]

Aha, I see you're looking for help (and possibly for a new maintainer).
Perhaps, I can at least, help in keeping things updated and take care of your baby in the best way I can! 😄

That would be greatly appreciated :)