Drop git ls-files in gemspec

[utkarsh2102]

Hi @pboling,

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:

➜  rspec-stubbed_env git:(master)  rubocop --only Packaging

Offenses:

rspec-stubbed_env.gemspec:25:5: 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.
    `git ls-files -z`.split("\x0").reject { |f| f.match(%r{^(test|spec|features)/}) }
    ^^^^^^^^^^^^^^^^^

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>

[pboling]

@utkarsh2102 First of all... I love your changes. Second, ship this in debian?! Third, I love Debian. Fourth, how /why did you find this, and how/why is it useful to Debian?

[pboling]

LGTM

[utkarsh2102]

Hi @pboling 👋

First of all... I love your changes.

Thank you so much! ❤️

Second, ship this in debian?!

Hehe, yes! So we maintain oauth2 in Debian (as a Ruby package) which now depends on this awesome library! 🔥
So we packaged this, too (as all dependencies of a package should be packaged, too)!
This mean, you can do: apt install ruby-rspec-stubbed-env on a Debian system to install this gem (alternative to gem install rspec-stubbed_env, but system-wide 😄)

Third, I love Debian.

That is awesome! And Debian loves you ❤️
Thanks for your awesome work on this and oauth2 (and other libraries that you maintain!) 🔥

Fourth, how /why did you find this, and how/why is it useful to Debian?

So in Debian, we maintain a lot of applications and libraries, one of them is oauth (and now, oauth2).
Since oauth2 started depending on this, we packaged this library for Debian, too! 😄

I'll be happy to answer more of your questions if you have more!? \o/

[pboling]

@utkarsh2102 So very cool. I am working on releases. 💯