Drop git ls-files in gemspec

[utkarsh2102]

Hi @tarcieri,

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:

➜  ed25519:(master) rubocop --only Packaging

Inspecting 14 files
..C..........

Offenses:

ed25519.gemspec:17:24: 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.
  spec.files         = `git ls-files -z`.split("\x0").reject { |f| f.match(%r{^(test|spec|features)/}) }
                       ^^^^^^^^^^^^^^^^^

14 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>

[utkarsh2102]

Oh wait, something's wrong with lint, that's weird..
So I am marking this as WIP and will ping you once it's worked out.

[utkarsh2102]

@tarcieri, fixed, CI is green again! 🎉

[tarcieri]

Thanks!