Set minimum Ruby Version to 2.5; add and apply Rubocop

[frostmark]

Closes https://github.com/assaf/vanity/issues/374

• In the next PR I will format the code. Code was formatted via rubocop

• I don't remember where I took this RoboCop config, but it was something popular ruby repository, and still use it everywhere. @bensheldon feel free to add something if you have some ideas Took config which @bensheldon suggested

• Also, I think would be nice to setup CI Rubocop Configured CI for Rubocop

• Set 2.5 version as minimal required

[bensheldon]

@frostmark here's the Rubocop config I use for GoodJob:

https://github.com/bensheldon/good_job/blob/2edcd0268ed2718c799776735e597339c2cf34aa/.rubocop.yml

I also wouldn't object to using StandardRb either, though I prefer inheriting it through Rubocop rather than using it on its own. Something like this: https://gist.github.com/gavvvr/5873313c6a87ba1d32ea31920d0557f4

I'd like to see this checked in CI, and I think we should run the cops/get-to-green as part of this PR so we can review the results of the rules.

[frostmark]

@bensheldon Can you take a look, please?

[frostmark]

@bensheldon ping ping 😄

[bensheldon]

@frostmark thanks for the extra ping! This looks excellent to me. 🙌

I intend to merge this, and then bump the major version to Vanity 4.0.