Update RuboCop

[AlexWayfer]

Require its specific version in Gemfile.

Fix it's installation in CI.

Enable new cops.

Resolve new offenses.

Drop support of Ruby 2.3
(it's not supported by RuboCop and by MRI:https://www.ruby-lang.org/en/news/2019/03/31/support-of-ruby-2-3-has-ended/)

Deprecate Faraday::Request#method, replace it with #http_method.

Todos

List any remaining work that needs to be done, i.e:

• Tests
• Documentation

Additional Notes

It requires for neighbor pull requests.

[iMacTia]

Thanks for taking a stab at this, we totally needed an update.
I've left some comments/change requests

[olleolleolle]

Thanks for your awesome work here. It's doing a lot of good!

I had one question, you can find it inline.

Thanks again!

[iMacTia]

Thanks @AlexWayfer for addressing all my comments 😄!
And sorry if I came across as a bit fussy, we really appreciate all the improvements you've added here!

One last request (but not mandatory to merge, I can do it later if you don't have time), since we decided to rename #method into #http_method and deprecated the old method, it would be good to update the UPGRADING.md file as well to keep track of all backwards-incompatible changes.
I think this is going to be the first v2.0 note!

[AlexWayfer]

And sorry if I came across as a bit fussy

No problem.

we really appreciate all the improvements you've added here!

Thank you, I see. 😉 And thank you for help and discussing.

One last request (but not mandatory to merge, I can do it later if you don't have time), since we decided to rename #method into #http_method and deprecated the old method, it would be good to update the UPGRADING.md file as well to keep track of all backwards-incompatible changes.
I think this is going to be the first v2.0 note!

I agree, added. If you want another content of this note — please, write, it's not too hard for me to change.

[olleolleolle]

Looks good!

Thanks for sticking with it. <3