Drop `git ls-files` in gemspec

Utkarsh Gupta requested to merge utkarsh2102/linter:use-rubocop-packaging into master

Hi @lienvdsteen,
Thanks for this gem, this is awesome! 🚀

However, the downstream(s) face difficulties when there's git usage in gemspec for listing the files. This could very well be avoided by using a pure Ruby alternative, as this MR does!

Additionally, the gemspec has been tweaked to only ship the files that are actually needed and also fixes the spec.execuatbales field 😅

I hope this makes sense and you'd be okay with this 💯

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

Merge request reports